That really stinks! I thought that because cyprus mulch is wetter it would help keep any traveling mites at bay (again, what the same person at that one pet store told me).
The store I go to doesn't keep their substrate near their reptiles... but it's a Big Al's store (if you have them/have heard of them), so the whole place is reptiles/amphibians/fish, so I wouldn't doubt that at some point - maybe in the back room - they had the bags next to the animals. Good point!