Its freedom of speech. I can hang a Nazi Flag in my yard if I wanted to. The stars and stripes has a long history of slavery and murder on its belt but I don't see people calling for its removal (give it time though). This makes me want to buy a stars and bars to hang in my yard. Anything the government says I cant have just makes me want it more.
I'll never understand this virulent strain infecting modern conservatism, doing things just because they're perceived as angering someone else. It's like wasting power on Earth Day, or "rolling coal" when encountering a Prius: what's the point?