The number of people signing up to drive tells us a lot about the number of unemployed / underemployed individuals who have a car.

A job that's slightly better than starving to death is not a good job.

The best job opportunity for them is the best job opportunity for them. I wouldn't blame Uber for the fact that their driver jobs might not be some idealized version of the best possible job. For many of the drivers, it presumably is the best possible job they can get. I'd blame every other company who couldn't do better, not the one company who seems to be giving the best opportunity.