You must not be familiar with Fred Phelps and his crew. They're the ones that picket soldier funerals and have the website godhatesamerica.com. That's why they'll burn a U.S. flag.
As to why they'll burn either one to begin with, there's apparently a relationship between Westboro and the group in Florida. Media whores of a feather, flock together, that kind of thing.
The most irritating thing about these clowns is the fact that they get the attention and the ground zero Mosque imam is issuing threats without being challenged.