Agree with you on the passing on of such books. This underlining etc... makes me unhappy as I view it as destroying the book.
A feature on Kindle was / is listing how many people underlined a passage of the book as I'm reading that part. Turned this option off.
Think Lub correct in ascribing most to college undergrad quick reading for a course and then selling to used book store. I once acquired the Fred Dustin book 'The Custer Tragedy' for a really nice price. This literally had underling and margin notes on almost every page as well as pictures of Custer, images of troopers, flags, and poetry pasted in the margins throughout. Nice price or not all this was a distraction which rendered the book unusable to me. Though clearly a prized possession of the previous owner.