It occurs to me that maybe pet stores and bike shops have more in common than you might think. In bike shops, the big ticket items are bicycles; they're usually the most expensive single item a customer will buy, and they're theoretically what the store is "all about". However, bicycles are not at all what bike shops live off of; that would be accessory sales. Bicycles are big ticket items, but selling one requires a fair amount of an employee's time (and the better the shop, the more time the employee takes). Bikes don't get that great of a markup (percentage-wise), and those that don't get sold before the next model year is released usually have to be marked down (even if the next model year has changed little beyond the paint color); not to mention that with a big-ticket item there's additional pressure to compete with internet sellers making far less than typical retail margin. So bike shops more or less break even on actual bikes, if they're lucky.
What lets them keep the lights on is accessory sales: clothing, helmets, locks, cupholders, shoes, drink mixes, seat covers, racks, fenders, bags, etc. Also regular maintenance items like tires, lubes, chains, cables, etc. Repairs can vary, but they're still practically never a cash cow.

In a pet store, the actual pet might be the "big ticket item" if it's especially exotic, but animals have to be looked after, which cost money and requires time; and selling a pet also requires a good bit of an employee's time (and similarly, the better the pet store, the more time it takes). If the animal doesn't sell, it still has to be fed. I don't know what kind of markups pet stores get on animals, but I'm guessing they aren't cash cows either. So that leaves accessories: enclosures, decorations, heaters, lights, collars, beds, etc; plus maintenance supplies like food and bedding.

I've worked in the bike industry, but not in the pet industry, so please correct me if I'm wrong. But in the bike industry, what that all means is that if you value the advice and expertise, or even just the convenience, of having a good local bike shop, you might as well buy your accessories there instead of online, even if it's a few bucks more (but you'll have it right away and won't have to pay for shipping, and you'll be able to see it before you buy it and know for sure that it's what you wanted).
And I would imagine the same is true of pet stores. If you appreciate having a high quality pet store with knowledgeable staff that takes good care of their animals, you might as well support them by buying accessories there.

This is all assuming that the place has what you want and that they're actually good - there are crap bike shops that sell people bikes that don't fit and crap they don't need just like there are pet stores that don't take care of their animals or don't help their customers be prepared to do the same.

But there are some definite parallels, no?