Because locking the phone to their SIM card means that the carriers can charge international travelers enormous rates for minutes and data rather than just putting a local sim card into the phone in each country.
I also believe that carriers shouldn't be allow to lock phones. I don't think carriers should be in the business of selling phones in the first place.
It appears that most of these things aren't actually necessary and T-mobile is pulling back the curtain to show what ATT/Verizon are doing.
Really, why? I don't care for the latest overhyped bells and whistles, my current phone is an SIII Mini which I bought for an affordable price, and even better, I put whoever SIM card is best for me at the time