I frequently carry a can of soda or a package of snacks into the movie theater. I've come to see this differently. A monkey could do the work. A valid ethical precept must work not just for one particular case but also for all similar situations. For an agreement to be meaningful, it must be entered into freely by both parties, hardly the case when a theater owner stamps a list of rules on the back of a ticket.
Rules ought to be reasonable.
And there is a difference between the rules imposed only through the authority of private property and the laws devised by a democratic society, which at least strive for the consent of the governed and are nominally under the control of their representatives. The latter have more legitimacy than the former. Commerce, too, should be freely entered into. And if fewer purchases at the concession stand means a theater has to raise ticket prices, so be it. That is also a part of commerce.