I've never heard of that either. But I can guess it's meant to shield the requesting library for financial liability if the patron never returns it. If they're on the hook for replacing the book, then...

And actually, there are a number of academic books I've had to request through ILL because they're only in a handful of libraries, the initial print run from the academic press was probably 500 at most, and replacing one would probably cost $1,000, simply because there's only one person in the world currently with a copy to sell (if you're lucky), and they can basically set their price.