If he declares now with a movie premiering on HBO this weekend that is two hours or however long the movie is, the network has to give every presidential candidate the same equal amount of airtime free. So if a Law & Order episode with him in it, that is one hour of airtime the networks have to give all the other candidates, as well as any movie he has appeared (Hunt for Red October, Days of Thunder and so on). Therefore, the last thing the networks want to do is give free airtime to the candidates because that cost them big $$$$$$$.