In times long past, Kingdom Halls had insurance policies, just like our own houses and paid for under the hall's normal operating funds. Then as more and more Kingdom Halls became 'owned' by the society, the idea was floated that an insurance discount from a worldly firm was now possible because of the size of collective contract. But once all the halls were fully under the control of the Organization is term of being 'self insured' Kingdom Halls because a thing.
But really when the Society say 'self insured' what they really mean is, that they will take the risk and pay for the repairs themselves.
However when a hall is damaged or destroyed, the Governing Body may pay for the materials, but the labor is very often obtained free through the local RBC. Thus in the end, a type of double saving is made ... and maybe more if the congregations around the damaged hall are encouraged to donate funds for that project or the world wide work.
It not a scam ... but seeing the rebuilds continually done by local Brothers, when enough money was supposedly set a side to pay for an outside contractor, makes me believe that it is a policy decision.