Small business, Single owner: "Can I get a doggie bag", thank-you and leave...maybe go back in, sans gun and carrying an educational pamphlet....Still, it's his place, he can make his policy on the spot....I respect his right to do that.
Regional Business: Happy to comply, are you the store manager? If not, can you call him? I'd still comply with the request and ask for his name if the Mgr wasn't around....Also, May I have the number of the Regional headquarters (along with your name)....oh, and can you put this stuff back for me, I think the ice cream is melting.
National Franchise: Happy to comply, is this the store policy? Do you have a copy of it that I may see...Are you the store manager? If not, could you call him?... if not available, I get his name and ask for the corporate offices number, maybe ask if there is a Regional Manager number...Oh, and can you put this stuff back for me, I think the ice cream is melting.
I see little value in trying to convince a local mgr (underling), that he is wrong even if it's true....I'd just leave the cart, contact his chain of command and then follow up with them on what their policy is (asking for a copy). After I confirmed, I might go back and visit the person again and make an effort have a reasonable conversation....(ie: "Going Rogue in Albertson's: A local Store Managers story" - although, I don't seem to have had any issue in the stores I've been in).
Anyway, That's also assuming I think all this monkeying around would be worth it, or just go back to Safeway, Fred Meyers or WalMart.