Excluding goverment buildings and schools it's the US CONSTITUTIONS call ! I'am amazed at all the bars , dr's offices and amusments parks and etc that think they can stop you from open carry or concealed permitted carry . I guess no one has challenged their rules ?
You may be amazed to find they have the right to prohibit carrying a firearm in a building they own or control, the same as I have the right to prohibit someone from wearing a firearm in my home if I so desire. If you do enter a place that prohibits carrying firearms and don't leave if told to leave, you may be amazed to find you can be prosecuted for trespass.