Having helped out at events many years ago I know that back in the day some contracts, depending on the guests, had either side of the event clauses where they could not promote a specific show or attend other events for a certain period of time. It is pretty standard for any contract in what ever industry where the employer wants to protect his or her investment. Heck, my contract at work states I cannot get another job alongside my current one in the same industry as it would be deemed a conflict of interests.