I mean if the Pats weren't going to give him the opportunity for small minority stake and someone else did, why should he not pursue his business interests? Brady also has some influence in the Raiders front office and gets to be part of team building.
I get disliking that he's connected to another team, but the truth is, if it was a problem for the Pats franchise, they easily could have been the team to do this. They didn't want to. Neither of them owe each other. Brady gave enough to this franchise and Kraft paid him his contract. Now that it's over, they can make their own decisions.