Yep, I used to work for a YMCA summer camp, and we actually questioned not having seatbelts or other safety-restraint devices on our buses for the kid's safety, and was told by the rep from the bus company (who was giving us a while briefing on school bus safety that was really quite interesting) that there has been testing done to passengers in bus crashes, both with and without restraint systems, and the "people" that did not have a restraint on actually fared BETTER in the crash than those that had restraints.
They called the system "compartmentalization" and basically said that when the bus hits something, you slide forward on the seat and more of your body mass (usually arms, head and chest) hits the seat in front, spreading the impact over a larger area, and the impact is also taken differently, which makes it easier to take the strain, while if you're wearing the typical restraint, a lap belt, you swing forward at the hips, and your head is the only part that makes contact.
Our buses had seat belts in the first three rows for those parents who had concerns about their kids riding the bus and needing them to be buckled in...but no parent ever used them.....
So, saying all that, basically, you don't need your seat, and that is why
Tim