This is a dog whistle. What they're really saying to white people is "stop using this term because it's making POC question our practices".
White people don't refer to themselves as 'racist' any more. Even white nationalists have stopped using it. That way it's easier to pretend that racism doesn't exist. Any Black person who calls it out is either wrong, promoting an agenda, or making excuses for poor performance.
Right now there's a massive government push to make drivers employees. If that happens you'll be able to sue the company instead of just the individual racist driver. Obviously Uber doesn't want that to happen. Billboards like this give them plausible deniability.