The ethical debate here is actually a great example of the difference between the two separate definitions of stealing: illicit benefit vs. deprivation of resources.

On the one hand, if stealing is defined as depriving others of their stuff, you're stealing next to nothing from the movie theater by sneaking in: a fraction of a cent's worth of wear and tear on their seats and carpets, the water you flush down the toilet, and the opportunity to use the seats you're occupying in some other way (essentially worthless in a show that isn't crowded). On the other hand, if stealing means benefiting from someone's stuff without their permission, you're stealing $10 worth of services here.

This is one of the two basic disputes about intellectual property: when you do something someone else did first (e.g., print a copy of a book or use a particular manufacturing process), you are depriving them of nothing, but you're benefiting from work they did without compensating them for it. It is impossible to deprive anyone of intellectual property, but it may still be wrong to appropriate it for yourself if they have a legitimate right to forbid you to do so.

Incidentally, the argument that the majority of the cost of your ticket goes to the distributor is actually a strong argument in favor of screwing theaters (if you're into that). After all, if the movie theater is just passing on the screwing the distributor gives them, sneaking into the movie is mostly just "hurting" the distributor, not the theater. Practically virtuous!

My own reluctance to put this into practice comes from a different direction: I must be honest about buying tickets when I go see Evil Brain-Sucking Hollywood Movies, but only as a disincentive to see them at all.