The customer is at fault. Assigning blame anywhere else is simply the "pass the buck/not ME!" crap that contributes greatly to what's wrong with our world.

If you buy a set of pots and pans, do you blame Wal Mart when the food you cook sucks?

Besides.... how long does it take to learn very basic animal husbandry? While that question is obviously rhetorical, the answer is also obviously "way longer than you're going to talk to the clerk in a pet store."