> The mere presense of a gun shouldn't involve calling the cops
Unless it's against the terms of service of the private corporation
At least in the US, police don't enforce terms of service of private corporations. That's a civil matter, enforced by filing a lawsuit.
If that was truly the only issue then it is a civil matter and the cops have no jurisdiction without a ruling from a judge. If I walk into a store with no shoes despite them requiring shoes, the first action isn't call the police and have the person arrested without saying anything.