If you see people walking into a store or a restaurant with openly-carried rifles and weapons, there is no way for you to ascertain what their intentions are. You can’t tell whether they are a “good person with a gun”, a “bad person with a gun”, or a “generally good person with a gun who could be triggered by some unknown level of confrontation into becoming a bad person with a gun” or anything in between.
The correct action to take is to remove yourself from the situation. Abandon your cart of purchases, don’t worry about paying for food; just get out. It is perfectly rational to feel unsafe, and it’s better to be out of the way in case their (unknown) intentions are not good ones. You should not feel obligated (legally OR morally) to stay and pay for your purchases if you feel your life is endangered. If you are in a restaurant and have already eaten your meal, you can call the restaurant after the fact, let them know why you left, and offer to pay for it over the phone. You may call the store and explain why you left your purchases in a cart in the middle of the aisle. You may also advise them whether you intend to return to the restaurant or store if you do not feel safe there.
It’s a risk with which companies that allow open-carry into their stores and restaurants will have to learn to deal.
Unless you can explain to me why I should always feel perfectly safe when random people carrying rifles walk into a Target store or a Mexican food restaurant or any other place where rifles are not part of the general commercial activities conducting within, then this seems to be a perfectly rational response. This isn’t even a gun-specific action: I would probably do the same thing if someone came into a Chipotle wielding a chainsaw.
Then again, “Stand Your Ground” laws in your state also allow you to take matters into your own hands if you are armed and feel threatened, and act preemptively. I can’t make any recommendation in that situation, but it seems to me that the law would be pretty clearly on your side in those cases.