Myth, it's symbolism per se, it's the confederate flag, with is a symbol of racism to a lot of people. And, as I've posted, if you use such symbology for a avatar, then do not be surprised if some people, myself included, take issue with it. End of discussion on the confederate flag.
As for the crescent, it's a symbol for a lot of things, including Islam. It's a symbol of the moon, etc. But, I truly understand the outrage for using it for the Flight 93 memorial, given that it was downed by Islamist, who by the way, are not the same at Muslims. But, most people do not make that distinction, and will attribute it to the whole religion, and of course the Islamists who downed the plane.