Okay, I'm not sure how other people would react to this situation, but I remember being pretty pissed at the time.
Last summer, my friends and I started a Rocky Horror Picture Show shadow cast at our local theatre, which does midnight showings of the movie two or three times a year. We'd worked it out in advance with the theatre manager, and the understanding between us was that we would help clean up after the show (a major undertaking) in exchange for being allowed to bring in minimal set pieces (a table and two chairs) and some props.
We got to the theatre early enough to start setting things up. About ten minutes before the movie was scheduled to start, the manager came up to us and said, "Okay, guys, we're gonna have a sold-out show, so I'm going to need you to give up your seats so more people can come in."
There are several things wrong with this:
1) We did not get into the movie for free just because we were doing a cast; we paid for our tickets like everyone else, so it's not like the theatre was losing money on us.
2) Not everyone is onstage at the same time; just because we will be performing does not mean that no one is going to want to sit down, especially since a good half of the cast is wearing high heels.
3) Nothing about this was mentioned when we first set up our agreement.
In retrospect, the manager was probably also violating fire codes by cramming more people into the auditorium than they had seats.
If I hadn't been so preoccupied with getting all of our props in the right place, I probably would have taken the issue up with him, but as it was, we all sat on the floor during down time, which meant uncooked rice embedded in people's butts.