Uh, I think that depends on the nature of the contract between Google and T-Mobile. If the phone costs Google X to manufacture and they are selling it for less than X as part of a contract where T-Mobile pays them some portion of the difference, it makes some sense that Google would want to recoup that money if you cancel the contract with T-Mobile. There is no restocking fee here, you are keeping the phone, just not the contract.
T-Mobile ETF is almost a red herring here, phone companies charge that either way. And you paying T-Mobile ETF probably does not reimburse Google (my guess).