The tickets are for a game in a stadium owned and managed by the State of Washington. Hence the Seahawks are a quasi-public entity and bound by the same access rules that a government agency would be bound by.
I would agree with you if the Seahawks owned the Stadium, but they don't. It was bought and paid for with tax money.
That has absolutely no basis in the law. Yes, the stadium is publicly owned, but it is leased by the Seahawks for their games. As far as I know, the State of Washington has no role in ticket sales or policies, and does not benefit from such sales. The fact that it is a publicly-owned stadium does not mean that the team cannot decide who can/cannot buy tickets to the game.