Sorry if someone has already covered this
You would be suprised to know that many rental sites run without insurance.
They instead choose to rely on the disclaimer, which basicly isnt worth the paper it is written on unless it is backed up with proper insurance.
These sites seem to do allright for themselves and obviously are saving themselves alot of money. They are taking a huge risk which could leave the owner of the business in debt for the rest of his life.
Proper insurance as already stated can cost thousands and also be alot of work (you need a proper risk assessment, safety pricedures trained staff, first aid, the list goes on) but alot of sites that are doing well at the moment are running with no insurance at all!