I would think that if you don't inform them you'd cause delays for the other 40+ people on the bus.
I imagine they must have a list of people that are supposed to be on the bus, and will check you off when you board. When it's time to leave and they're missing folks they have 2 options: leave anyway and hope that the Smith family isn't late because little Timmy had to make an emergecy run to the bathroom, or wait and try to find the Smith family or hope they show up.
If they leave anyway, they'll put themselves in a position of having a very angry Smith family calling CRO and complaining about the incredibly poor Disney customer service and how they're now stranded at the airport. If they wait then they're inconveniencing everyone who was on time and has already boarded the bus.
However if you let CRO know that you won't be using the service then it a) takes care of the above situation entirely becaue they'll know you aren't going to be using the service and b) opens up the spots for other people to use.