Ostensibly, the multi-decade vision for Uber is that it becomes not just a taxi substitute but the operator of all the transportation in the future, as people abandon cars in the US and as motorized transportation reaches more of the third world, eventually culminating with them having a good chunk of the ownership in the driverless-car market... and that's where the value will come from, not just the disruption of the entrenched taxi interests (which is only adequately lucrative, and, as you mentioned, riskier). And regulatory diversity at least ensures that there will be somewhere that they should be capable of proving out a model with which to tempt the rest of the world.
Realistic or not, I tend to think that the management buys into that kool-aid.
Airbnb, that one is a bit murkier. There is something to be said for people wanting better value for money, and being able to rent a nice apt for the price of a small hotel room surely is a good thing. Even if you would add some sort of hotel tax to that, I'd say it's still superior value to a traditional hotel.
Hypothetically, there's not really any reason to put scare quotes around the word right: At its core, every company is a vehicle for delivering value to its current owners. Future owners are not current owners, and the only defense mechanism they have is good old-fashioned caveat emptor.
In a well-functioning system, that would be sufficient: Few companies would follow the "pump and dump" scheme because it would be so difficult to pull one over on prudent, skeptical investors that approach would rarely meet success. I think, though, that we are not seeing a well-functioning system. We're finding a system where companies inevitably flow toward less and less savvy investors, until eventually you've got half your family members talking about how they're thinking about getting in on the Facebook IPO based on nothing more than that they keep hearing in the news that it's selling for a lot of money.
Incidentially, I was terminally amused by all the news articles talking about the "failure" of the Facebook IPO. On the contrary, if I were one of Facebook's pre-IPO owners I think I'd have been laughing all the way to the bank. Only in what passes for popular financial news could someone do the equivalent of characterizing a masterfully executed Poker bluff as a woeful failure without being rightfully dismissed as someone who's terminally clueless about the rules of the game.