But in the case of a used book store, the number of copies of the work does not change. For someone to gain the work, someone else will lose it. You are in effect giving up your right to that work.
There can be non-profit motives as well. Let us suppose I have a twin sister who dies after a long battle with cancer. I write a book about that battle. Ten thousand copies are made and wow, they are incredibly popular. But I had hoped that book would alleviate my pain. It makes it worse though. Now, suppose I, not the publisher, hold the copyright. They want more copies of my inspiring book. But I don't want any more published. I refuse. The sale of a book at a used bookstore does not injure me in any way in such a case. Ten years later the pain has subsided and I feel comfortable publishing it again. I make a million copies and become rich, set up a foundation and that finds a cure cancer. But suppose some yahoo makes a pdf of my book two years after it goes out of print. Now I can't make a dime off of my hard work.
Same with LUG's books. Same with FASA's books. Or West End Games. Somone still holds the copyrights. What if Decipher wants to republish the Way of D'Era. I'm not sure who holds the copyright for that. Suppose it's Wizards. Suppose Wizards sells copyright to Decipher. Or if Paramount holds it they say, "Ok we changed our policy, you can use it".
Decipher reprints the book, changing the LUG stats to Decipher Stats. But, if some yahoo goes ahead and publishes a PDF then Decipher doesn't really have a chance to make money off it, do they? Maybe in ten years Decipher will wind up reprinting it. Maybe in twenty. The copyright is just that, a right. You don't have to print it. It is your choice. As it should be, it is your creation. Your right includes the right to make it difficult for people to get your work, whatever the reason.