Let's first off deduce that, interns are received into industries and facilities for the basic aim of acquiring more knowledge from the facility.
I understand some employers use that as a privilege to extra-work their interns which is bad, and in that case, I think it's mandatory for that intern to be paid. But when the sole aim of the intern, which is learning is being met adequately without bias, do you think the government or the schools that sent their students for internship programs, demands that the employer pays them...? Let's say you are the employer, will you, under duress or some form of laws, pay people that come to your facility to learn from your wealth of knowledge? Will you in the first place accept them?