I am not sure what protection under the law has to do with anything. Sneeka2 did not mention anything about protections, only the stupidity of Uber's maneuver. Posting a private key in a public place is pretty dumb. Not revoking and changing your keys once you discover the mistake is also stupid. Expecting someone who finds the key to not use it is also stupid.
The things you mentioned are also risks, to different degrees. I don't leave my car unlocked with my wallet on the street. I find that stupid. I shred any paper with my social security number on it. I find putting such paper in the trash stupid. I still expect legal protections in case I do make a mistake, but I would expect that if I left my car unlocked with my wallet on the seat that someone might burgle it. That's a pretty reasonable expectation. It's why I avoid exposing myself to such risk.
Private keys posted on the public internet are dumber than putting a wallet on the seat of an unlocked car.