I know they want to be and that's the disgusting part. They want far too much control. If I have to use a locked phone I want some freedom. One thing I do respect Apple and its iPhone for is maintaining control on the phone... that and I get to watch ATT beg you to get a Lumia or Galaxy SIII, and hear them whine about iMessage.
Thinking of that, I have no idea who provides my parent's landline phone now, Ameritech, SBC, or ATT. I was a youngster but I do remember getting long distance telephone service with Sprint meant you could hear a pin drop. The HD voice teasers had a pin on them and I got all excited and people looked at me like I was insane.
With all the big phones launching on all the big networks, poor T-mo, carrier loyalty is plummeting and people will switch. Too much restriction could be very costly in the long run.