As I noted above, equal protection enters the equation in a "place of public accommodation" - a stadium (along with restaurants and hotels, most notably) is specifically designated as such. There's a more stringent guideline for places of public accommodation beyond the protected categories, in which it is unlawful to refuse service (i.e. refuse to sell tickets) for arbitrary reasons; there needs to be a legitimate business reason to do so. In this case, the argument that there is a legitimate business reason is that more home team fans make the team more likely to win a game and teams that are win more games achieve greater revenues, both of which you can almost certainly show strong evidence of in the data. Therefore, there's a very strong case that it isn't discrimination, but the suit isn't as frivolous as it looks on the face.