Honestly, it's just the way things work. It's not specific to this phone or provider. You get a subsidized phone when you sign up a _new_ plan. It's part of the cost of the service you're agreeing to. If you're already a customer, in the middle of a contract, they don't "reward" you by subsidizing a phone any time you ask for it (i.e. when a new one comes out).

When the N1 came out, however, people were for some reason surprised about this. There's lots of news about it: http://www.google.com/search?q=t-mobile+%22nexus+one%22+%22n...