Whenever someone says a rule, it's generally important to follow it if you can... but it's even more important to figure out why the rule is in place.
In this case, the manager doesn't want you to say, "Geez, that new Flooby movie absolutely sucks" - and a potential movie-goer overhears it and decides that maybe they shouldn't come and watch Flooby next week. The manager doesn't want your comments to hurt sales.
Once you understand the reason behind the rule, it starts to make sense: your coworkers didn't actually do anything wrong. Because there's a huge difference between:
- "Geez, that new Flooby movie absolutely sucks"
- "Geez, I can't stand horror movies."
Why? Because someone hearing the first might not see Flooby. But someone hearing the second isn't going to be dissuaded from seeing a specific movie. It's not like someone who's a fan of horror movies is going to have their genre taste irrevocably altered by an overheard comment.
Branching off from there, I'd suggest two general rules of thumb:
Be cautious when questioning someone with experience. I'm not saying someone without experience is often wrong, or that someone with experience never is, but... there might be reasons why they're doing what they're doing - that they have experience to judge a bit better. In this case, they might have internalized what I suggested above: it's okay to poke fun at a genre, but doing it to a particular movie might hurt sales. Whenever you see someone with more experience doing something that you're not, take some extra time to figure out why they might be doing it. Sometimes there won't be, but sometimes like this case, there will.
Give people a chance to be good. Let's say they were insulting a specific movie, that they were doing something they definitely shouldn't be doing. Confronting them directly might not be the best approach. Instead, I would simply ask, "Are we allowed to talk bad about the movies? I thought we weren't supposed to criticize them at all when we were on the job?" - aka, give them the chance to say, "Yeah, you're right" without polarizing them or putting them on any sort of defensive.