This will give you an idea
https://www.msauk.org/assets/405-409...ppendix2-1.pdf Excess is is payable by the organising club and normally £425, however obviously Mull was considered to be a special case and presumably the amount asked for by the insurers was deemed too high for the club to risk. Remember that clubs are mostly small and volunteer lead. In theory they could provide in the regs that competitors would be responsible for the excess, but if an incident occured then it would be up to the club to first pay the excess to the MSA/insurers. They could then try to recover that money from the competitor but that wouldn't necessarily be a quick process. If the excess was, say £5,000, then there's a chance that the club wouldn't have the kind of money required to actually pay.
You also have to remember that the current 'climate' of rallying in the UK is in a pretty precarious situation in regard to liability. The incident on the Jim Clark in 2014 (and subsequent inquiry) mean that the insurers are particularly jumpy. I suspect
It's a UK publication so would angle everything towards UK fans, hardly surprising.