This is about Petland not Petco. The Petlands in the greater NYC area count the number of people who enter the store. It doesn't matter if you buy anything or not. They then calculate how many people enter the store as a way to price and promote the store and also what price to charge for all live animals and all other merchandise.
An active store brings in more live animals, raises prices. So even if you don't buy a thing you are still helping the store to prove to the franchise chain that it is a busy store and can sell more live animals and merchandise at a higher price. So if you care about animals sold in a pet shop don't even walk in the door !