It's an ages old story with real artists. First, a few people notice them and realize they're something special. When everyone else starts to notice and offer them big money and lots of attention, quite often their art suffers and becomes ... crap, basically. They start making what they think the general population wants to see/hear. This is selling out. U2 never did that. No matter how many people loved what they did, they strove to create something better. They never changed the meaning of their music to please anyone or released twenty songs like With or without you just because it was popular, they continued to grow as artists and maintained their integrity. This is why I love them.
Other people think because they changed at all, this means they sold out. It's just artistic growth, in my eyes. A lot of people just don't think you can have integrity and commercial success at the same time