I also saw that exact prerequisites for being surveyed was ownership of a U2 iPod, replicas of Bono's Fly glasses, a distaste for Coldplay, delay pedals, and a Vox guitar amp.
Honestly, if you don't believe a marketing research firm because it doesn't feel right to you, then fuck it. No one cares anyway.
All I can say is, U2 360 was the biggest tour of my generation. HTDAAB along with American Idiot were some of the most talked about albums during high school. And it wasn't uncommon to see Elevation tour T-shirts in the 7th grade. I'm only 25, and I've been a fan of the band since ATYCLB. My girlfriend, who predominately listens to alternative music like Bon Iver, Fleet Foxes, Lana Del Ray, and M83, thinks Every Breaking Wave and The Troubles is the shit, can't stand Mysterious Ways, thinks the Joshua Tree is just okay, and insists U2 in Under The Blood Red Sky is the coolest fucking musical event she has ever seen on video. If you think U2 is limited to a certain audience, then you are just ignorant to proclaim so. There are preteens making SOI fan videos on YouTube. They probably don't even know who U2 even looks like.
U2 always had fans across the entire age spectrum, and that doesn't make it surprising that a 1/4 of all active iTunes listeners listen to them, especially considering THEY GAVE AWAY A WHOLE FUCKING ALBUM TO EVERYBODY ON ITUNES. Good god, most of them probably are just listening to Every Breaking Wave because the song is fucking fantastic.