Chipotle says leave your guns at home

[Gman]

Then explain to me, if it's their "right", why can it be taken away by a private business entity? Doesn't the law allow this company to set the rules for carry on their premises?

The property owner can't deny you a right. They can however deny you access to their space or limit who can exercise their rights in their space, but to do this, they must follow the law explicitly by signage or they can ask you to leave. Anyone complying with their request to disarm is voluntarily agreeing to the arrangement. An individual can also choose to exercise their right and avoid that property.