why are internships legal?

Because an intern should not simply be providing "free labor." An intern should also learn the day-to-day workings in their chosen field. An internship should be just as, if not more, beneficial to the intern than the employer. If an "intern" is merely doing tasks that could be out-sourced to a temporary (paid) employee, then the employer is likely violating labor laws.

In the US, many interns are paid at least minimum wage. And, in fact, some companies pay their interns quite well. Google and Microsoft have been known to pay over $6000 a month for some interns.

But, there are also many interns that can work with no pay according to US law. There are some requirements, but companies can hire interns for no pay. But they do usually get college credit for it.

Because if you work for free you're not technically employed, that's why. You have to be paid to be an employee, and if you're paid, it must be at least minimum wage.

Because newbies need the experience to get their foot in the door.

It's up to that person,if they can acquire experience,and knoledge to expand themselves,then so be it.

because if it wasn't people would complain that the government was oppressing them by not allowing them to work for free if they want to.