Why? Because they pissed off the Westboro Baptist Church by writing Keep it Clean (Hot Buns), a song about gay sex. Fred Phelps decided that his merry band of arseholes would go to the Foo Fighters’ concert in Kansas City for one of their famous pickets.
In response? The Foo Fighters came out, on the back of a lorry, and performed Keep it Clean for the protesters.
Keep it up, Foo Fighters. Keep it up.