On any public bus transportation there are no seatbelts. Local city bus, highway-going city-to-city bus, etc. As many have mentioned, the buses around Disneyworld do not have seatbelts either!
I'm like many here...absolute carseat person when using a personal vehicle (or shuttle with seatbelts at
Disneyland), but for public-type transportation, I trust the physics of it...compartmentalization I've heard it called?
But we used carseats on the airplane for DS's years of being 2 and 3 (after a miserable no-carseat-on-plane trip when he was 17 months, I will NEVER do the lapbaby thing again!), mainly to keep him contained AND to protect against sudden, HUGE, turbulence.
With a bus accident, it's not out of the blue. Turbulence, however, IS, quite literally, out of the sky (often clear blue, LOL). I feel that with a bus, things might happen slowly enough that I could react, but with huge turbulence, I can't. So with a little one that might be wriggly in the lapbelt, I would prefer a nice 5 point harness that they know to stay in...hence, carseats on planes, but no worries about public-bus-type transportation.
My bigger concern is any fight I might encounter having my 19 mo old rear face on the plane! (We bought the seat in front of him as well so it doesn't restrict another passenger)
You might want to check on the Transportation forum, for the rules on rear-facing. The use of carseats on a plane are for entirely different reasons than in a car....rearfacing is to help with collisions and the forces those impart. You're not encountering that on a plane, or if you do, it's going to be the least of your concerns.
I personally would expect a FA to say "no" to rearfacing with such an old baby...tiny infant I would expect a "yes", but not a 19 month old... And there's really no point to it, for the plane.