There are no car seats allowed on any bus that I know of. That includes school buses, yet we still send our children on those; right? I was just making the point that children are on buses many of an occasion without a carseat. ME is a big coach bus. I still bring carseats on the plane to keep my kinds a bit more secure. My DS2 is a houdini with seatbelts but hasn't mastered his carseat harness yet. I will stowe it under the bus for the ME and then in the closet at the hotel. That way, IF we end up renting a car for a day I have the carseat just in case.
Actually megabus has seatbelts and they reccommend that any child over the age of 2 and under the age of 7 be in a car seat. Oddly enough under the age of two they want them on your lap unless there is an extra seat available.