I thought there were no guaranteed contracts in the NFL?
Most teams have similar clauses in their contracts. The question becomes, who would want to enforce those clauses? The press would excoriate any team which actually sued their fans, and it would be disasterous for public relations.
It's unlikely to happen here, with the long waiting list for season tickets, but say 10,000 ticket holders decided to break their contracts, and there weren't takers for the tickets. A team would be stuck with not only thousands of dollars in lost revenue, but that many more tickets to sell to avoid blackouts.