"Someone came in with their elderly dog that just dribbled wee everywhere. I saw another dog sniffing at the wee and then weeing beside it too. Dogs compete with other dogs for wee supremacy. Also, dogs may get aggressive with each other, even if they're fine with humans.
In a pet shop it's yucky enough; in a supermarket totally inappropate."
True. I work in a vets and clueless people stand watching their entire male dogs sniffing at the side of the doorway on the way in, the corner of the reception desk, the shelves with food bags on and the legs of the consult table ALL THE DAMN TIME, then express surprise (if they even notice at all) when the dog naturally lifts its leg.
We're usually on the ball to point out the pre-pissing behaviour so the owner can stop it, or at least handy with a mop, blue roll and disinfectant spray, but some of this is inevitable. We're also very handy at knowing which dogs need to be kept away from other dogs and doing dog tetris to keep everyone safe.
A increasing, ever changing throughput of dogs down narrow supermarket aisles with loads of vertical surfaces and owners whose attention is on what fabric softener they want to buy is a recipe for piss splatters everywhere.
Plus as previously noted, it's not fair on people with allergies. Fine to have some pet friendly pubs and cafes, especially if people have a choice of dog free ones, but not essential shops.