Anytime moose are a factor in the production and presentation of any art, special considerations must be made. 

Many of us, myself included, use the airline ticket comparison to theatre seats.  Here's the problem I now have with it:  at a certain time, the door shuts and the plane leaves.  If you're late, you can't run down the runway and expect the plane to stop and let you on board!  The theatre seat, on the other hand, is still accessible no matter how late the patron is. 

And I don't think we're going to be better off to think of theatre tickets more like hotel rooms.  Although they aren't going anywhere, there may be penalties if you try to cancel a reservation and once you paid for the room, they don't care if you stay in it or not. 

One suggestion I have is to leave it up to your patron.  Let your subscribers know that there are often people who don't get to see shows and if they are not going to show up, please notify the theatre.  In this day and age of cell phones, even a moose detained patron may be able to call and say they're running a little late.  (I don't want to ignore the possibility that if moose are involved, there is the issue of remote regions with no cell phone reception.)  But patrons may also call and say they can't make it and give you permission to re-sell their seats.  In return, if you are an established non-profit organization, you can offer them a tax receipt for their donation. 

What is important here is what someone has already pointed out:  you must be clear about your policies and they must be written.  My bottom line on this:  That seat has been "rented" for that show and if the "renter" chooses not to use it, that's their business.  But they may choose to "sublet" and you can be there to help them with that.