Any property owner or business manager can say that someone is trespassing on their property or premises for whatever reason.
The point is that they have to tell that person that they are tresspassing and then also tell them that they have to leave the property or premise.
It's not illegal to walk on the property with a firearm, but if they tell you to leave and you refuse, they can file charges to have you prosecuted for tresspassing.
At least that's how it works in Texas.