Is it really that few? My perception was that many more caves required PLI. In the north, Fairy Holes, Birksfell, Boreham I think all those agreements mention BCA insurance. The likes of Easegill, Leck and Excalibar require you to be in a BCA member club at least. Renegotiation of some arrangements on Ingleborough and Penyghent in recent years have removed the requirement for PLI or national organisation membership and this is set to continue.
I can understand the requirement for a show cave business as cavers may well be mingling with the paying tourists. Otherwise it seems a belt and braces approach by the landowner/management group.