I suppose really it doesn’t make a fig of difference to the cinema at a cost level so long as they’re not generating four screens’ worth of cleaning-up or taking up seats in a sell-out screening.
The only revenue loss would be if those ‘scrumpers’ would otherwise have come back to watch the other films as a paid viewers if they couldn’t sneak in, or whether they’re really going to see the one they wanted to see and a couple of ‘supports’ that they weren’t fussed about. And conversely I suppose, would those people have even bought a ticket in the first place if they were only limited to one film instead of getting a few for their money?