Allow me to attempt to be thorough. Here is the issue:
Was it right for AT&T to block access to 4chan?
There are a number of ways that one might address it, since "right" is rather open ended. So I will address the issue from a multitude of perspectives: business (that is, profit), business ethics, legal, and desirable.
From a pure profit standpoint, we may discard the questions of 4chan's actions. It does not matter if any sort of criminal activity is going on; as long as it is profitable, it is good. So, would AT&T's actions have benefited the company financially? In the short term, yes. The activities of the site are of no importance; what is important is the amount of traffic that accesses the site, particularly the traffic that utilizes AT&T's infrastructure. The more traffic there is, the heftier the infrastructure has to be, which turns into costs (both to implement and maintain). By no longer accepting a significant bulk of traffic (what we'd normally call a DDoS attack), AT&T would relieve strain on their resources. Short term, it saves them money. Long term is too complex for me to feel comfortable commenting; it depends on the public’s reaction to the situation. It might well result in a decrease of profits over the long term, however, if this causes consumers to find alternate sources.
From a Business Ethics standpoint, we may still largely ignore any questions of the legal nature of actions that occur on 4chan. Rather, AT&T has made a contract with its customers. AT&T will provide a service, the customers will pay for the service, and everyone is happy (ideally). The service in question is access to the internet (through the infrastructure that AT&T provides). AT&T's actions violate that contract; whereas before their customers had access to the entire internet (and that was the service for which they were paying), AT&T swapped out the product. They wanted to only provide access to part of the internet, but were still charging their customers for access to the whole internet. In essence, the contract was that AT&T would provide service A and the customer would pay for service A. But under the new model, AT&T was providing service B while the customer was paying for (and expecting) service A. Bad business ethics, thus it was the wrong course of action. Furthermore, assuming AT&T was primarily concerned about the infrastructure stress associated with DDoS attacks, their attempt to reduce strain by limiting access to a site is really just their attempt to reduce the likelihood of consumers from utilizing the services that they have purchased. The strain on the infrastructure came from multiple users (in this case, user computers) accessing their paid-for services at the same time in an inconvenient manner. By restricting access to 4chan, AT&T was attempting to restrict what was, from an internal limited perspective, legitimate use of paid-for services.
From a legal standpoint we may now address the issues of dubious behaviors. Such behaviors, assuming they are not housed on AT&T servers, are outside AT&T's realm of concern. They can neither say that such behaviors are legal or illegal. If they know that a specific individual is engaging in illegal activity, they are quite within their rights to notify the authorities and refuse to provide additional services (taking away services that the individual has already paid for, however, becomes highly dubious and could be considered stealing). If AT&T was targeting specific individuals who they believed were responsible for illegal activities and if they merely denied them further services (instead of taking back services already paid for), then they might have legal justification. However, their actions were not limited to criminals or potential criminals but a broad, multifaceted swatch of the population. This could reasonably be considered to be a violation of anti-discrimination laws: AT&T could no more deny service to the groups of people who wish to access 4chan than they can deny service to women, or blacks, or homosexuals. This is, admittedly, a broad interpretation of consumer laws; one would need an actual court case to determine if it is too broad. Regardless, the legality of AT&T's intentional discrimination is dubious, at best.
So then we have the question as to if it is desirable (regardless of profits or laws) for AT&T to restrict access to 4chan. From a consumer perspective, no. No restriction on consumer freedom is desirable. From a business perspective, no. No limiting of one's potential market is desirable. From an objective standpoint, we could potentially say yes. 4chan is hardly a bastion of civility; it is curled up and died, I doubt the world would suffer (and it may well be a better place for it). However, even if we say that 4chan should not exist in an ideal world, there is still the question as to if AT&T restricting access is desirable. Given the above, even if access should be restricted, it would seem that AT&T being the one to restrict it is undesirable. And finally, it is desirable for information to be freely available and easily accessibly. This concept is what allows humanity to develop; any restriction to that concept is a restriction to humanity itself and I will no truck with it.
Your standpoint, ToD, is that, as a private business, AT&T is free to provide what services it desires to whom it desires, correct? Unfortunately that is not the reality. AT&T is limited by anti-discrimination labor and consumer laws; it has to provide services equally to all parts of society, within reason (I doubt anyone could sue because AT&T doesn't provide internet access in the middle of Death Valley, for example). The key is on if they are free to provide what services it wants. To an extent, yes, but once it has already committed to providing a service, it is bound to providing those services or returning the money. Terms of Service agreements may at times be changed without prior notice, but the services themselves cannot be. All customers should have been notified of AT&T's change and offered the opportunity to end their service immediately and receive a pro-rated refund. Furthermore, in the future, they would have had to avoid false advertising. As it is assumed presently that businesses that provide internet access to the entire internet, AT&T would need to specifically note that their service is different and only provide access to a portion of the internet.
