How would she even know if her customers owned an AR-15? 
From The Washington Times:
A Maine restaurateur is facing calls for a boycott after she announced she was banning owners of semi-automatic rifles from her restaurants, as well as people who support the private ownership of such rifles.
Anne Verrill explained in a now-deleted Facebook post why she’s banning some gun owners from her Foreside Tavern & Side Bar in Falmouth and Grace restaurant in Portland, Raw Story reported.
“If you own this gun, or you condone the ownership of this gun for private use, you may no longer enter either of my restaurants, because the only thing I want to teach my children is love,” Ms. Verrill wrote, including a picture of what appeared to be an AR-15, similar to the Sig Sauer MCX that Omar Mateen used to kill 49 people at a Orlando gay club over the weekend, the Portland Press Herald reported.
“You don’t privately own this weapon to protect your family, or to hunt. I understand that I may be offending members of my community, but this is a human issue, not a gun owners issue, or a Second Amendment issue, it is about humans,” she continued. “I cannot, in good conscience, accept anyone inside of my restaurants who believes that this is OK.”