MLB definitely uses the avg/yr for the purpose of figuring the lux-tax, probably as a hedge against avoidance due to creative accounting.
As far as it being called a $28mil contract even though he'll never get that figure, I doubt there's any MLB rule which requires them to announce it that way and, while I don't know that they chose that figure so as to make it known that it was the highest per/annum deal ever, there's so much bullshit being slung up here that it wouldn't surprise me if that were their angle.
Roger, btw, was asked twice in the first hour or so if the deal included the provision where he could opt out of travelling on trips where he's not on the sked to pitch. The first time he avoided the question and didn't give an answer. The 2nd time he answered; "I have no idea", adding that he hadn't yet read the contract.
Like I said, it's flyin' pretty thick these days.