I'm intrigued about this...
By extenuation, could the argument be made that - if the bus had been nearly full, able-bodied people should have been made to leave th bus to enable the wheel-chair user to access a wheel-chair slot?
This is one of those areas where legislation will just make matters worse, by more clearly defining what "good manners" should explain quite clearly, and remove any use for said manners as a result.
I personally think it's sad on both counts: that the mother didn't move (or attempt to move, I suppose?) and the disabled person felt the need to sue.