In the article it says Baker is the one who is doing it not Thompson. Baker is the one who wants Thompson to run and started this whole thing. If Thompson runs he will get sued by NBC who have already stated they will do that. But Thompson is an attorney and he might know some loopholes about that sort of thing. His best buddy is McCain who he might not want to hurt by running. And he did promise his wife when his daughter died that he would never return to politics. But like I said all that could have changed right now.
I'd tell NBC to sue my ass off. When I'm elected, see what if any contract GE gets or if NBC even gets remote access to the White House!
I hadn't really thought of it before now, but Thompson might have a bigger issue even if he declares after the current season wraps: re-runs. Even if the pld episodes featuring Thompson don't air on the NBC broadcast network, but they run anywhere from 1-4 hours a day on TNT and USA, and having to pull them over equal-time concerns would cost NBC-Universal, the studio with the syndication rights, a ton of money.
That said, I don't see (disclaimer: IANAL) how NBC can stop him from running for office -- UNLESS they have a specific provision in his contract where he promised not to run for elected office. In that case, it would be a straight breach of contract, and NBC could sue for the revenues they lost as a result. That wouldn't completely stop Thompson from running, but it could take a bite out of his campaign funds.