I don't think that the sites are exploitative, I just think they have a very defined niche. In Brazil, for example, you are considered to be "middle class" if you are making between $291 - $1,019 USD per month. In Pennsylvania, I wouldn't even be able to move into a very cheap apartment with $1,000 a month, and would really struggle to do so with $2,000 a month.
So sure, if you live in the US and you are trying to underbid people in other countries, where expensive of living is so much cheaper, you're not going to find yourself making a reasonable income, most likely. But if you are just trying to build up a portfolio, this could be a decent way to try to make a few bucks in the process.
Likewise, the type of work available on these sites are generally locationally irrelevant, which makes it an outstanding way for anyone to access these sources of income worldwide.
So no, it's just the market speaking, so to speak. It's just a lot of people aren't used to having such a diverse competition.
This is why so many jobs go oversees. Not just because people are "willing" to work for less there, but because, sometimes, a smaller income has a comparable power where these workers live.
Obviously, that isn't always true, and many people are exploited worldwide for cheap labor.