In many states, security guards can be armed in public, but so can private citizens.
The endgame for this will be large-scale gated communities that are town-level. A surefire to prevent homelessness in a particular town is for the entire town to be private property. If you can't card-in, you don't get in. Dystopian yes, but probably inevitable, and the trail was blazed by the modern corporate campus that operates on the same principles.