Putting up a sign announcing a policy does not guarantee people will follow the policy. That is the lesson for the owner. Just as the texter also did not follow then rules.
The lesson for the texter is follow the rules when informed and don’t grab someone’s food and throw it at them.
Those are the lessons.
especially if your name is Curtis Reeves and you are a retired cop who believes himself to be above the law and not bound by a posted notice that the rest of the unarmed peons in the theatre must adhere to.