Here is my guess as to the real reason those school buses have no seat belts: there is of course always at least one adult on the bus to supervise the children. If the bus had seat belts and one of the children were injured or killed while not buckled in, the employer of the supervising adult would face legal liability for neglecting to ensure that the child was buckling in.

Specifically, a complaint that the bus should have had belts can be met with the reply, "But that's the way school buses have always been." In contrast, there is no similarly-reasonable-sounding reply to the complaint, "If only the supervising adult had done his job and ensured that the kid was buckled in!"