Except Uber doesn't dictate how you use the app, they set parameters on how you can use it. They also don't tell you that you HAVE to take all rides, but if you do not meet a quality bar or are abusing their product then they can kick you off it / revoke your access...

The only difference here is the payment. And to that end, if Outlook supported the ability to transfer money then it still wouldn't make Microsoft your employer...