That is utter nonsense. Absent some provision in their lease agreement requiring the Seahawks to sell to all comers, there is no requirement to do so. Private companies do not become quasi-governmental entities simply because they lease public property.
I'm shocked that Freepers would support forcing a private company to sell their product to customers the company does not want to serve. Particularly where the compnay could make a strong argument that doing so would harm the company (stadium loudness/etc. has become a significant part of the Seahawks brand).
How long have you been an attorney?