I'm guessing there are a lot of people with money who just don't want to deal with carrier bindings.
Actually, its cheaper to buy an unlocked phone if you want to use t-mobile. Compare the 500 min + unlimited txt + unlimited data (although slow) for $60/month tmobile plan to the AT&T 450 min + unlimited txt + 2GB capped data for $85/month. That $25 difference pays back the subsidy in 18 months, less than the 24 month contract, plus you have an unlocked phone which means cheap local plans overseas instead of the outrageous AT&T international plan.