I agree with Neal. While it may make you feel better to scream at someone (and it's hard not to even for adults), in the end it doesn't do you or the snakes in that store any good. Instead try talking calmly, if that doesn't work (which in this case it didn't seem to), then ask for the manager's name and the store number. Contact the corporate offices for PetCo and lodge a formal complaint both by phone and in writing. In that complaint indicate the conditions you found, the date and time, the name of the manager and the store number. Also let the head office know that you will be returning to the store in a week and if conditions are not changed, you will be contacting the animal control authorities to report them.
A calmer, controlled plan of attack will likely work better for the future. If you get kicked out of the store you can't go back to check on the well being of the snakes, so in the end nothing will get done. I understand it's hard to stay calm but it's best if you can.
I'm not doubting you but are you sure what you did see was mites and not speckles in the snake's pattern? Just checking.