As trivial as you may consider their argument, if the potential benefit of installing and enforcing the use of seat belts on school buses is small enough, it may not take much justification to do nothing.

In the US from 2003-2012, there were an annual average of 6 fatalities of non-driver occupants of school buses[1].

[1] http://www-nrd.nhtsa.dot.gov/Pubs/811890.pdf