They are most likely in a pimp/prostitute type situation where they have to collect their money from the driver they are impersonating and are only getting a % of their earnings. Being an Uber driver is already a lower paying job, so only getting a % of that in Miami, a very high cost city is certaintly exploitation.

Then again they are illegal immigrants so it might also be the best opportunity they could find. I sure as shit would rather drive people around in my car then pick produce in the strawberry fields we have down here.

They're getting a job, but it's cheap labor and aren't being paid a fair wage.

I suppose in prostitution there is illegality on all sides, but a prostitute could still be exploited by her pimp.