They are not a public entity, they are private business and as such can make the rules for their properties.
A municipal building or a public street would be different. We the citizens pay for those things, and as such we the citizens set the rules with a vote.
Now USAA being a private company stockholders own the private company. As such they can make the rules for their company. The only ones entitled to change the rules are the owners.
If a private citizen or a customer doesn't like it they don't have to go there or be a customer anymore. The ruling in Heller did not change the rules (and rights) of ownership of private property.
And smoke nazis.
The Heller Case has nothing to do with this. This is effected by a new Florida State Law enacted by the Florida Legislation that says an individual who has a concealed permit can have a firearm in their vehicle parked in their employers parking lot.