Enforcing contractual obligations isn’t unconstitutional. We don’t know what the contract actually says but it might plausibly include damages for cancelling in these circumstances.

Edit: With that said, the center isn’t helping its case by claiming things like

Your dismal ticket sales and lack of donor support, combined with your last-minute cancellation has cost us considerably. This is your official notice that we will seek $1 million in damages from you for this political stunt.

Calling the action a political stunt may allow the defendant to argue that the lawsuit is motivated by retaliation against speech even if the lawsuit would otherwise be valid, and admitting that ticket sales were dismal isn’t great if you’re going to seek damages for lost ticket sales.