Well of course they use some backing on some songs---who doesn't? Most bands have to, and it has nothing to do with legitimacy, only with the fact that it would be hard for the Edge to play strings and guitar on kite, or pull out some brass for Stuck in a Moment, or work the synths on Streets, etc, etc.
But there is a huge difference here: U2 make and play all of their own music (well, of course, they have their producers, but you know what I mean). They actually play live. Bono is really singing, Larry is really drumming, Adam is really playing bass, and the Edge is really playing guitar. A random drum machine intro or rhythm guitar part added in for a specific song doesn't take away from that.
Ashlee doesn't play an instrument, doesn't write her songs (IMHO), did not work to get into this industry at all, and lip synchs completely. That this is compared with the fact that U2- like most bands- add some parts with tapes/additional guitarists on certain songs, is a little absurd. I understand you saying the superiority complex is kind of ridiculous, but the fact that they are even considered in the same realm of existance baffles me. She is not an artist, IMHO.
But she is a "pop artist" who was designed to make money and make people feel all happy inside---and I understand her place in the industry. There's nothing wrong with listening to a pop song if you like it, some are pretty catchy. Just please don't pretend like she has as much integrity as U2!