Goodell said that he fined the Patriots as a team because Belichick is effectively the team's general manager as well as head coach and exercises so much control over the Patriots' on-field operations that "his actions and decisions are properly attributed to the club."
Goodell imposed a fine on the Patriots as a whole because Belichick acts as both the team’s general manager and head coach, exerting significant influence over on-field activities. Consequently, his actions and choices are considered representative of the club.