There's nothing worng with them checking your water. It's standard
procedure when you return a dead fish. If your chemistry was off they would
make you fix the quality of your water before you could take home anymore
fish. It is good business on their part and it makes perfect sense to me.
The only down side to Petco is that some of the people working there aren't
knowledgeable. This also varies. Anyway, where else are you going to get a
15 day fish guarantee on fresh and salt water aquaria. Not likely. It
sounds like you were annoyed that they didn't take your word about your
water quality. Put yourself in their shoes.