That's a lame legal argument by NBC. People (and companies - which are "legal persons" under the law) waive their Constitutional rights all the time through contracts. If they're arguing that it wasn't a valid contract that's fine, but there's no mention of it in this article. Next time they better think twice before issuing an invitation and then taking a candidate's acceptance.

Bigger question is "Whatever happened to the journalistic principle of "equal time", and "Why don't we have publically funded elections?"