I just read through it, and T-Mobile's deal is basically a 0% APR loan with a down payment and fixed $20/month payment, on top of your monthly service charge, for however long it takes to pay off the principal (depends on the price of the phone).
Any competent lender is going to provide you with a contract which spells out what happens when the loan ends, what happens if one or both parties terminate early, etc, and in T-Mobile's case, the loan is contingent on maintaining carrier service, and the remedy is full payment of the balance. Otherwise, people will just quit and get a $600 phone for the price of a $99 down payment.
Similarly, most new auto loans may be contingent on maintaining a service of some sort, like full coverage insurance. I think Washington State's AG has his head firmly implanted betwixt his butt cheeks, since any non-retard should easily tell the difference between the pay up front no-contract, month to month deal, and the other one which includes all kinds of disclosures as to the fact they're agreeing to a loan... But whatever.