The state of Texas actually has no interest in making the playing of the national anthem compulsory beyond red meat for the rabid, psychotic base. Of course everyone knows this. Even Republicans know this. After all, patriotism is the last refuge of a scoundrel. I would add that patriotism is a particularly powerful groupism by which much agitprop has been created to destructive ends. Thus it truly isn't the last refuge but far too often it is the first refuge. Wave your flag, wear your freakin' stupid flag pin on your lapel, "these colors don't run" (Oops, please see Ronnie Reagan and the bombing of the barracks in Lebanon) etc ad nauseum ad infinitumI don't disagree that there's nothing particularly patriotic about a professional sporting event, particularly one that pits two teams from the same country against each other. I think in the rest of the world, they only play anthems in international play (competitions between national teams of two or more countries) or championship games.
It has, though, become customary in North America and that's fine, it's a free country. But good lord compulsory national anthem playing is definitely fascist-y. Technically its a statutory contract requirement, if you don't have a contract with the state, you don't have to do it. But these days the state is entangled into all sorts of venue-management, incentive, and other deals. What legitimate interest does the State of Texas have in making the playing of the national anthem compulsory for its pro-team contractual partners? This is just more red meat bullshirt.
Query whether a new state requirement for state contracts can be written into existing contracts by statute, I suspect it cannot. In other words, the law only applies to contracts signed after the statute's effective date. It may also raise First Amendment issues, though the contract framework is different than a broadly applicable law, I'm not sure how that works.