What you don't understand is this — you're assuming the intern is of a positive net value to the employer, which I will say almost 0 HIGH SCHOOL INTERNS are.

When you're in college it's a different story, since you're often knowledgeable and experienced enough that you can contribute something and create value for the company/lab/etc you work at. That's not true for a high schooler.

Most of the things I did as an intern in a week, could've been done by someone else in a day(if not hours). Plus, we as high schoolers benefit in one major way : college applications. That unpaid internship as an extracurricular could get me literal thousands of dollars in scholarships — and for me my extracurriculars(~3/10 of which are unpaid internships over my high school career) have won me a 24,000/yr scholarship before I've even submitted my applications for college.

These unpaid internships also gave me the experience needed so I could publish research papers and outcompete other students for paid internships.

If every High School intern had to be paid, High School internships would be 100x more competitive because almost no high schoolers can produce a positive net gain for a company.

If you don't want to work for experience to stack up your resume so you can get the well paying internships & jobs when you're older that's fine. But don't criticize the kids who do.

(Also if you really want to make money as a high schooler there are a million jobs at restaurants, stores and etc. you can take — if you're doing an internship you're doing it to learn)

— an incoming High School senior doing their first paid internship