Fine, but don't call it a gratuity. Call it a service fee. "Mandatory gratuities" make me look for another service provider.
Agreed with this, but I can get over it for now.
If I understand this properly, it is a gratuity. The way I understand it, the taxi is still a normal taxi in operation but essentially has hooked into Uber to give it a new way to find fares and to have people pay. If this is the case, then the rate is the same with or without Uber and you still need to pay the gratuity and Uber just automatically selects that gratuity to be 20%. It's actually appreciably different from Uber choosing a rate and then adding a 20% automatic gratuity on top of it.
They make you rate them, so you have to do that interaction anyways. Not really much more inconvenience if there was a slider or buttons there for tip %.