It's always debatable how truly mainstream U2 is.
On one hand, you have a band that sells millions of albums, wins Grammy awards, is on TV, sells out tours around the globe, is on magazine covers, has hit singles, etc., etc., etc. Clearly this would give anyone the impression that this is a big-time mainstream group.
But on the other hand, a quick listen to U2's music and it's clear that they never really fit in the mainstream - in fact, sometimes they are the ones creating the new mainstream! JT, in 1987, sounded like nothing else out there. AB, in '91/'92, was the opposite of grunge rock. AB had very thought-provoking and dark lyrics, but it also had more upbeat styles in music and sound. It stood out in an era of grunge. In '00, ATYCLB was an incredibly fresh rock sound when bubblegum pop solo acts and boy bands dominated the scene. HTDAAB has a bit of competition with Green Day (for the punk elements) and with Coldplay (for the more classic U2 sounds), and a few other bands like The Killers and Franz Ferdinand and Snow Patrol - i.e., other artists clearly influenced by U2. Still, at this moment in time, this is but a handful of artists in an era dominated by rap and R&B. Yet despite this, U2 is still forging new ground, creating new sounds with songs like "Love & Peace" and "Fast Cars".
So when one considers how "on the edge" U2 is musically and lyrically, one could hardly call them "mainstream".
As such, U2 is a conundrum - wildly popular for not producing songs that fit in with what's considered wildly popular! Hence it doesn't surprise me if the indie crowd shows some respect.