Sources who are usually quite chatty re: rumors are paranoid A.F. when this subject comes up. Which leads into my second point ...
Being somewhat familiar with contract law, I agree with this to an extent. But everything about it -- the obvious secrecy, the odd choice of only one day (probably) at the last possible moment, a solid source saying WB 86ed the house months ago --- something unusual is going on here. This whole situation screams trying to exploit a loophole in a contract -- were this a TV show or movie*, I'd suspect to extend rights, but Halloween mazes are a unique medium so who knows?
* Best example of this is probably the pilot made for a "Wheel of Time" TV show that was aired, without any sort of advertising, at like 1:30 a.m. Just so the producers could keep the rights to the book series a few more years and hopefully try making the series again when they had more money. This just feels like something along those lines.