No doubt this will end up in the SCOTUS if they choose to take it that far.
Freedom of speech.
I have no doubt this is being driven by a large Somali muslim immigrant community.
No, a security guard is not an employee of the government. Its a private corporation and can do this sort of thing without worry about the Bill of Rights, which is only a protection if the government is trying to infringe upon the enumerated and protected rights.
What should happen is that the security guard will be fired or posted somewhere else if the security is handled by an outside contractor.
This will only happen if there is a large enough outcry from the community. Otherwise, nothing will be done.