I remember looking at that site before. I also remember talking to an agent that spewed the garbage about narrow axles. It was all I could do to not verbally shred them on the phone.
The main reason cited for this is the axle of a school bus is generally a narrower axle than that of a regular bus.
At the same time, there have been more than a few rigs on the net and here that push the limits. Roof raises of 2-3 feet. Roof racks that are meant for people. Tanks being held on with ratchet straps, etc. It's no wonder that the companies shy away from insuring skoolies when they see some of the pictures. You want them to take a few hundred bucks in exchange for tens of thousands in coverage. I agree that an off the shelf school bus is safer that a stick and staple RV, but the RV is a known commodity. Who knows what crazy stuff a skoolie will come up with. Think about it when you build your bus.