Every band is a "sell out"... they are trying to "sell out" seats
I never heard any fall out boy music but the term sell out is a dumb one when it comes to musical acts... a band isn't one persons personal possession, they are a business trying to stay in business by gaining as many customers as they possibly can. I support any decisions my favorite bands make in order to "stay in business" and when it doesn't work, they will try something else.