It's basically what you said. Buses are generally, on most city roads, the biggest vehicle on the road, are highly visible, high off the ground, and have large hard seats to protect against impact. The reason we wear seatbelts in cars is because of how many people were killed in vehicle accidents that could have been prevented by wearing seatbelts. That same thing is not true about fatalities on buses. Not very many people die in bus crashes every year, so we don't need a law protecting us from it.

Now, passenger buses on the highway are different. You are up against larger vehicles (trucks, other buses, etC), BUT I will say, if something happens and that bus slides off the road into a river or lake, it's going to be a lot easier to evacuate everyone if everyone is not strapped into the bus...but obviously there is a trade off there. Most Greyhound type buses do have the seatbelt option available anyway, so you can choose which you would rather risk. (I'd rather fly.)