I'm guessing it's part of the contract when the fleet owners hire, as well as something the owners watch like a hawk. Since the drivers wish to keep their job in order to keep earning, they theoretically need to worry about ratings as much as the next person. If we assume that finding a new fleet owner is easy, then jumping ship and finding employment with someone else may make the driver's need to maintain a good rating a fuzzy matter.

If anyone wishes to learn more, here's an article to throw into Google Translate:

http://www1.folha.uol.com.br/cotidiano/2016/03/1746900-motor...

Simple; all drivers register their own Uber account, and lose the car if their ratings are not good enough. Actually I wouldn't be surprised if the cars were rented out for a flat daily fee instead of depending on the number of rides.

Just a taxi company using Uber.

No they are complete crap, worse than taxi drivers