Whenever a shampoo bottle is "empty" my wife will put it on top of the shower enclosure so that she will see it later and throw it into the recycle bin. What I do in the couple of days before she actually removes it, is use it some more. I take the cap off and fill it with about half a cup of water, then I swirl it around to dissolve the remaining product. This method will usually give me a couple of hair washings.
The fact that I can do this is proof that she does not do it and she is pretty smart*--Masters in Biochemistry after all. In fairness, she has much more hair on her head than I do and so it takes much more shampoo to wash. The remainders are plenty for me but probably wouldn't make a dent in what she needs.
This "shampoo conservation" as I call it may seem insignificant, but it all adds up: If it saved one bottle of shampoo over a lifetime (and it probably saves more than that), it would save 300 million bottles of shampoo in the United States alone. Prices are pretty variable, but this could represent a billion dollars. This is a drop in the bucket for an economy well-north of 10 trillion. On the other hand, I could live quite well for the rest of my life on 1% of a billion (10 million dollars).
Spread-out over a lifetime it shouldn't make a dent in profits for makers. So hopefully, nobody from this secret society
*She is also pretty conservation-minded: She will extract the last molecule of toothpaste out of the tube before throwing it out. It is almost a contest with us, who can get one last use out of a tube. She wins this pretty often given that my hands are much stronger than hers.