Whoever made that seems to have forgotten about freedom of religion.
Sorry dude, all I did was the demotivational. That one I google image searched for.
But I will admit, I did look for something a bit inspirational to go along with the flag, and that text from the NKJV seemed to fit that niche quite well.
Although in direct reference to your statement, I'd say that goes along with freedom of religion rather than against it. It's simply someone's method of expressing their religious beliefs along with their patriotism, which is their right. Were it a quote from the Torah, Quran or the Bhagvad Gita, I wouldn't be too terribly offended, even though I'm not a Jew, Muslim or Hindu, respectively.
In this case, that particular quote is often used when deferring pride to the military. It's more about courage and spiritual support of the soldiers who bear that courage, rather than about the Christianity itself. The entire first chapter of Joshua goes into this belief further, but verse 9 is typically the one that's quoted the most.
Anyway, I used that particular image because of its support of the military, not to upset anyone who doesn't believe the way I do.
If it does, just substitute "Lord your God" with "People," and you'll get the same, albeit more secular, message across.