In a perfect world? There isn't if it is willing.

The issue is that in the real world such a thing is way to open to abuse (even if say we see this kind of practice a decent amount, example: teenage babysitters).
Like for example sweat shops, and then there's also the matter of it's almost impossible to support one self for 3 dollars an hour. Back when it was legal to pay this much people would pay this little all the time and this was common enough and/or jobs were so hard to get at the time that people were forced to work their butts off for next to nothing because it was that or die.

I'm sure someone else will come in and say this far better than me (with far more info with it to boot), but in a nutshell it's something such as a simple job for a few hours for extra spending cash it should be fine as long as the conditions are actually humane. But if it's a job that's meant to be lived off of, then the person is quite simply not going to survive with such low an income and if they do, they just barely get by and leave nothing for their kids if they have any.