What Do Eyeglass Shops do with the Old Lenses When You Get Replacement Lenses?

OKay, I've asked this question to eyeglass shop personnel, and they tell me they throw them away. I am skeptical of that answer and I have my reasons. One- most won't volunteer to give you your old lenses back unless you ask for them. I've purchased many pairs of fairly expensive frames that come with high quality clear or sunglass lenses, like CR-39. You can't tell me they arent worth anything when the very shop sells them for $300-600. Many time I simply forget to ask for them back and now that I want to turn my opicals back to shades, I have to pay. A place I use alot, knowing I like to keep the old lenses of JMM frames - once I've had store personel bring the old lenses out, hold them and never oferring them to me as I usually forget to grab them on my way out. You mean to tell me that a store that sells JMM frames won't have use for JMM lenses? Do the shops throw the old lenses away, sell them to someone for recycling? I am hoping someone who works in this industry can shed light on this question for me.