It’s because AT&T is run by a bunch of greedy pigs and they want to punish users who want to use their crappy network. AT&T is too cheap to put up new towers and add bandwidth so they use caps as a deterrent. Between Pandora and Sirius XM I stream 50-60 hours a week on my Evo, depending on how many hours I put in at work in a particular week. I do about 5-6GB/month of data transfer on my phone. Sprint seems pretty committed to unlimited data, unlike like those hogs at AT&T and Craprizon, so no worries here.
So, if ATT didn't do it, another major company would .... then if the other company did , it would spread to the others anyways.