True statement-no sales to "green card" holders.
They implemented this policy after the Virginia Tech incident.
They do not feel it "is in the best interest of the business" to sell to anyone other than born in the USA.
"We reserve the right to refuse service to anyone for any reason" has been used frequently and it is posted IIRC.

Not sure what the legality issues would be, citizenship is not a "protected class" to the best of my knowledge.

I am neither defending nor endorsing the practice, just relaying my experiences.