I get around this happening to me because I have no knowledge of how retail pharmacy works. I dont know how to use the computers , enter a script, procedure for filling one, or any of the workflow .
So when a preceptor asks at the beginning of an IPPE "you alright with helping me a bit if/when things get busy", i just respond with "you want to train me? I've never worked in retail before" , and they usually just drop it. But if a preceptor tried to use me as unpaid labor, i'd report it to the school and probably have a talk with them about it quickly.