There is usually a 'corking fee' associated with this practice.
I think case with a coffee place, the "corking fee" would have been the purchase of a coffee.
What if you walk into Holt Renfrew or Harry Rosen with a shopping bag from Moores and you want to coordinate your Moores shirt and Moores tie with an Armani suit? Should the sales assciate go ape-shit over this or just let you buy the $3200 Armani suit to wear with your Moores shirt and tie?
Wait.... Moores and Armani... the sacrilege!
Since LAG was a paying customer, he should be allowed to do "whatever he wants".
If you deny service to someone just because they brought in something that wasn't sold in your own store, you can technically boot everyone out.
If LAG walked in with a Starbucks thermal mug, would Waves refuse to fill it just because they happen to sell their own mugs? Maybe not even one from a rival shop like Starbucks, but a mug from Superstore or something.
In Starbucks, they sell music CD's, so in theory you can tell people to leave if they decides to listen to their own MP3 player, since the music they brought into the store could have impacted their sales on those cd's.
T-shirts, boardgames, gift cards, a different credit card, etc... depending on how extreme you want to go, you can save money by not even having to have tables and chairs for your shop, lol.
Since it was only a bag of chip, it shouldn't have been an issue.... I believe the most you can bring in is like a sandwich. Obviously you don't start bring in your TV dinners and ask the restaurant to microwave them, or set up a 3-course meal on their patio, but if it's something minor, it shouldn't ever be an issue.