I know it's different hardware that makes up the network. So, let's give the boardroom the benefit of the doubt for just a second. There may be a cost to be recovered as the network becomes burdened by traffic that is a result of the higher bandwidth requirements for usage on a laptop when compared to a mobile device. If this is the case... DON'T sell me an 'unlimited' plan. (I'm in the USA).
I too believe it to be greed. Whether the above is true or not, I've known AT&T to behave this way for years. I was sworn off of AT&T since 2001, when I switched to T-Mobile after being screwed over, lied to and near-bullied. I just recently returned to AT&T only to have NetShare ganked from the app store. Crap.