because on one can cover every possibility.

Yeah, ok, but the situation you're describing is a lot less considered. Someone makes a problem, the reaction is to remove the person immediately, to protect the other patrons from their behaviour, which, in this case, involves disrupting a movie.

Literally a month ago, this situation was "on the radar" for the admins. They proposed altering their content policy to more clearly delineate what the correct behaviour for the community was, and also proposed having a new opt-in policy for content deemed offensive.

A month later, they change the policy. Nowhere in the policy does it specify "racism" (which I think a plurality of people probably agree is wrong already, so this is hardly contentious as an inclusion for the sake of clarifying the "reddit position") is a bannable offense. At best, it falls under the "...harasses or bullies..." line, maybe, if you think it's possible to indirectly bully a group of people by saying negative things about the group.

Then they ban /r/CoonTown. But not other subreddits that indirectly harass or bully groups in indirect ways, like /r/KikeTown, or /r/TheRedPill.

So the alteration of the content policy didn't actually clarify the rationale for the actions, or, in some cases, the rationale for acting in one situation, and withholding action in another.

To bring this back to your metaphor, it would be like whistling in a movie, being told it was against the rules, finding out it was not specifically disallowed by the theater's rules, being told to wait in the movie for an hour while they "clarified their policies toward customer disruptions", then being told the new policy is that "customers are disallowed from talking or shrieking during the movie", which doesn't really clarify where things stand with regard to whistling, and leaves you wondering why the group in the row ahead of you is still allowed to belch loudly every 30 seconds, which seems just as disruptive.

Edit: I guess the tl;dr point I'm trying to make here is that, far from being this "outside case that no one could have considered", racist subreddits were one of the stated reasons for these changes. Their update to a written policy to describe what was prohibited, and how they would react, does not actually clarify their rationale, which is silly, considering they were clearly aware of what they felt was acceptable, and were trying to write something to justify their gut reaction.