(USA) logbook endorsement requirement(s) for new glider type?

For as long as I've been flying anything, including and especially
gliders, I've had a CFI endorse a cockpit checkout in my logbook, for
any new aircraft type I flew.

Can someone tell me what/who requires this? Is it strictly insurance
companies, or is there a relevant FAR paragraph?

The reason I'm asking is that when I went to the local gliderport
yesterday to get a cockpit checkout in the LS-8, the on duty CFI
declined to sign an endorsement conducted by someone else, and he
would not do the checkout himself because he was not an LS-8 pilot.

I can't imagine that every CFI is able to personally conduct cockpit
checkouts in all glider types (with all due respect, Mr Compton).

The last six such checkouts I've received were performed by a non-CFI
and signed off by a CFI.

~ted/2n7o