You could book an Uber to two streets away and tell the driver that you actually want to go to somewhere else, and you'll pay the guy under the table. Bet they won't mind that.
Illegal. At least where and when I drove for Uber. And there were plenty of local government inspectors posing as passengers trying to catch these sorts of shenanigans.
Is there any reason why the law would be involved? Uber might not be happy but why does that have legal implications? Does this also mean they're not allowed to drive for anything else but Uber?
Ride hailing is regulated, in part, to ensure that people aren't being taken advantage of. It's also regulated to make sure that taxes and fees are being properly collected.
Taxis and private bus services weren't always regulated, but they became regulated because they took advantage of their customers. Similarly, regulations are placed on ride hailing to that passengers get what they paid for and there is at least some accountability.