The point is to take legal scenarios like this and see how far they can be pushed. What if her parents were the ones auctioning her potential earnings off and using the money to pay of a mortgage. What if the following year they did it again for another 10%? Things get really sketchy when you draw up these kinds of contracts that revolve solely around an individual.
Okay, so you're suggesting this seems to set a dangerous legal precedent with slippery slope potential? I still say it's just a loan. To your argument here, can her parents legally take out loans in her name? I don't actually know the law there, but I don't think a lawyer would have a hard time answering that.
To your earlier question about 51% or 100% of her salary, if an investor loans me $500,000 at 0% interest to pay back in 10 years, and I only make $50,000/yr, that would suck, but I'm not becoming a slave to that investor. I can't afford to pay that back, so I'm going to default on my loan. There's a huge difference between that and literally being owned by someone.
