You certainly weren't under any obligation to move. If you want the seats, get there sooner! Most of the theaters here have gone to assigned seating when you buy your ticket, so you don't have to show up early, but you do need to buy your tickets early as the prime seats for busy showings go fast! I hate that.
Before assigned seating, I did used to hate when parties would come to a very busy movie, say a Marvel movie on opening night, then want to leave one seat between themselves and the next party. It's like, come on, you know this thing is going to be full! At least leave a pair so a couple can take it.