The way buses are designed, the 'compartments' created by the seats are supposed to act as the restraint. Definitely not as safe as a five point harness of course. The argument is that buses statistically are far less likely to be in collisions than autos.
There is also the counter argument that the 'compartment saftey' thing only works on people of a certain height or weight. (I'm not recalling the figures off the top of my head, but they're definitely larger than the average kindergartner)
Personally, I would not be comfortable putting such a small child on a bus every time, but it is of course a parental decision