Kids applying to be interns in not the same thing than staff being unpaid and slave labor. That put clear limits on the duration of they work and that means they have no qualification. It's just a normal internship.

Three stars restaurant tend to have many interns because they have a lot of basic prep work to do and it's a good formative experience. That's fine. That's what internships are for. Usually interns are still paid a modest sum but the hospitality industry tends to operate on thin margins so they might be uncompensated. Where I kind of agree with you is that there is a general issue with abuse of staff in the industry so they are likely not treated as well as they should. Still I would not call that slave labor.

> A waiter in a restaurant I frequented regularly explained this to me as to how, despite working there for 3 years, he couldn't just go and work in the UK for a few months and come back because he could so easily be replaced.

Being replacable is not the same as being unpaid. Spain is part of the EU so the labor law of Spain has to adhere to the EU directives on labor. That means every employee is paid at least at the national minimum salary.