I managed several movie theaters (including a 20-plex) back in the day, so while doing this is unquestionably Wrong (in the “violating a specific implied business contract” sense of the term), I’d say there are mitigating variables that may not make it “Right”, but which people can use to more easily rationalize their decision.
The first and most important rule is that if you buy a ticket for one movie in order to get into another movie you’d otherwise be exluded from (usually because it’s Sold Out), you’re a grade-A jerk. This happened all the time when I ran the thearter, and having someone check the ticket at the door of the individual house to prevent theatre-hopping was an inconvenience for staff and paying patrons and a waste of resources (since that usher could probably be used behind the conecession stand, if the show about to start is Sold Out).
It’s not much better if you’re buying a PG-ticket so you can get into an R-rated film. I’ve found, anecdotally speaking, that the kind of kids who sneak into movies are also the kind who are also likely to be disruptive throughout the movie. Again, this usually ends up inconveniencing both the staff and the paying patrons.
Personally, I think the excuse that you want to see the movie but oppose lining the pockets of those who make it is risible. Within the industry, percentages and residuals and opening b.o. numbers are critically important to how money gets distributed–not only to the suits, but to the talent (actors, writers, directors, etc.). By consuming their product but paying someone else for the pleasure, you’re essentially depriving artists to moneys to which they’re entitled. That the same amount came out of your pocket (and subsequently misdirected) is immaterial.
If the proprietor of the local liquor store is a racist, are you in the right by shoplifting a soda and then leaving a dollar on the street corner? You’ve “paid” the same amount, but not to whom is entitled to the money.
Still, if you’re going to insist on doing this, then I would say the stand-up (if not actual “ethical”) thing to do is to buy a ticket for a movie that needs your financial support–movies that are smaller, independent, and represent movies that I, as a cinephile, would like to see more of at the local multiplex (and let’s be honest–this strategy only really works at a big multiplex). These movies usually have to jockey for screen space and often come and go to theaters in a flash if they don’t deliver immediately (it’s rarer and rarer for a movie to be given enough breathing room to find “legs”). So if you’re going to do this, then buy a ticket for Flight of the Red Balloon or Standard Operating Procedure when you go see Speed Racer or Made of Honor.
Just my $0.02.