i have to add that of course wasting books isn't good, but i'm pretty sure there is either some regulatory shenanigans at play, making it cheaper to just recycle them OR it is really cheaper and thus more effective.
publishers would LOVE to accurately determine how many copies they will sell while keeping people reading these cheap paperbacks happy. trashing the books is costing the publishers money. putting the extra books into recycling at least pays back a bit.
if someone does buy the extra copies for the same amount that would probably be fine as well, but there is a reason these books didn't sell - nobody wanted them in the first place.