Unfortunately these pet stores hire just about anyone, just like going to a electronis store (ie, circuit city, best buy, compusa..etc.) Most of these employees are just that, employees..its a job where they collect a paycheck. Everytime I go into those stores, I always brush off the salesperson because they're a waste of time to deal with.
Usually the smaller shops (mom & pop) type are more personable and knowledgeable in that they are probably into the hobby themselves and want to make money off of that skill/knowlege.
Petland is probably one of the worse out there. Most of them that I went to have a lot of people there that are in it for a paycheck. I forget what I was looking for, something for my dog and the employee said they don't carry the item, but I looked around and found it in the store.
That's why when my tank is set up, I'll order it online from That Pet Place or Dr.Fosters, this way I don't have to deal with these people. If anyone is in NYC/LI area, another good fish store is FISHTOWN USA on Northern Blvd, very helpful and knowledgeable.
I guess the owner of the shop only hires people with some background in the fishkeeping and are willing to learn.