As some of the comments here suggest, yes, there are people who will bring a cake to a restaurant to accommodate an allergy or to “honour grandma’s last year on the planet” by serving her “heirloom cake” (seriously?).  As a restaurateur I’m totally cool with that.  But, in my experience, most people who bring a cake do so because they are trying to pinch a few pennies and I’m not cool with that.  A restaurant is a business not a public service; it is designed to generate revenue and produce profit.  Every restaurant has bills to pay.  Anything that you, as a customer, don’t pay for is quite literally money out of our pockets.  Would you bring your own steak because you can buy a cheaper one at Costco?  We still have to pay our staff to serve your cake and clean up afterwards.   And every minute that a customer sits without consuming something they have paid us for is a minute that those seats could have been occupied by a paying customer.