Don't think a Uber car will be any more vulnerable than any other private car. I don't think they're externally marked. You summon one using the app.
A BG using a stolen phone might have some success, I suppose, but aren't most hijacked cars a spur of the moment thing?
If you are CC-ing and have to defend yourself, I think the law will overlook the trespassing while armed rule you might click on.
I think Uber put it in there to deal with potential insurance issues and costs.