mooseontheloose
See, moose love the Pats too!
So you are telling us to act like an ass because the clerk at Petco isn't a vet?
Not really any different than most retail experiences.
If you're in the know and have researched almost any topic well, your knowledge will almost always exceed that of a retail employee. As a PC enthusiast, I would never, ever go to Best Buy and expect some random employee would be able to provide proper advice regarding a PC/laptop purchase. Having worked retail, I know most are given just basic training to explain the products and provide a general idea of 'what's better than what'.
I'm guessing it's no different at Petco. Those guys probably have a general idea of which foods to stay away from and perhaps a few that are 'good', but whatever they push is likely based on marketing and information provided by the company (no guarantee that is accurate information). If you're uninformed and don't really care then you'll likely have better results than just guessing (same with my Best Buy example), but any semi-intelligent human could spend 30 minutes researching a buying decision and better understand their choice compared to taking advice from a retail employee.