With the first movie, Hasbro definitely would have had final say... but by the time the third one was being signed up, and paramount and Hasbro were grovelling to get Bay on board, all power may have been given to him instead by then. The amount of money Hasbro were making off the Movies, they would have done anything to keep the money rolling in, and would have just trusted that he wouldn't do anything irreversible or damaging to the brand, possibly giving him free reign and final (or even total) control.