Its not really that uncommon. Especially with students from India/China etc where their status in the country depends on the PI's good graces this can be quite

Its not really that uncommon. Especially with students from India/China etc where their status in the country depends on the PI's good graces this can be quite complicated. Students will literally do anything to make sure the PI is happy. The worst exploiters are professors from the same countries. They'll sometimes often only hire students from the same countries. I knew a case where a PI was caught screwing a student on the lab bench. She was dismissed, he was sent on a sabbatical.