It isn't? I'd say it's a very, very good reason to hold a grudge. If they force him to play for $1.5M, they're treating him like a rented mule for as far as any of us can see, no good reason. Any other QB in a contract year that went to the SB two consecutive years would be renegotiated, the only reason he is not it because of the size of the salary jump. Absolute bare minimum, they should renegotiate the last year of his contract for a 10x raise.
I fully reserve the right to change my position if it turns out he's being a tool, which is maybe not out of the question.