I find it to be a very sensible policy, disabilities regarding balance are not obvious, nor can the driver determine from his cab whether a passenger is struggling with shopping bags - concentrating on which means the passenger is not concentrating on his balance. Many a time I have been thrown into my seat, or someone nearby, when the driver does a grand prix start whilst I swap a heavy bag from left hand - to right hand - to seat. Perhaps buses in the US are more spacious than in the UK and allow shuffling of luggage whilst seated (or have, shock horror, front or overhead luggage racks!). Similarly when alighting bags need to be grouped onto one hand and then you need to proceed down the bus keeping a hand free to hold onto grab rails if NOT waiting until the bus has sopped. I find that the bus is delayed longer if I have to crawl around the floor picking up apples and oranges than it would be if I had remained seated until the bus had stopped - and my shopping thus not escaped from their carrier bags.
Having said that, some people take the mick by boarding then proceeding to have a conversation with several passengers before even looking for a seat, and then repeat whilst alighting. A comment from the driver, or other passengers is not always well received!