They do pay for their own education. They pay for it through substandard wages for the work they perform. If their department didn't pay for their tuition then

They do pay for their own education. They pay for it through substandard wages for the work they perform. If their department didn't pay for their tuition then there is no way those students would work for the peanuts they currently do. The net cost/income to the school would be the same but the current arrangement has less transactional friction and makes it very hard for the student to leave once they've started.