The people who take 15 or 20, I have found, sell them on eBay a lot of the time; for profit.
Personally, I take 2 to 5 of those left behind when everyone leaves -- as someone pointed out; they would've been taken anyway by the patron who left it behind. I take a few as I have some scrapbooks for different pictures, etc., keep one whole one for my collection and to give away others for friends who don't live here and are fans.
I used to usher Broadway shows, too -- and those stacks at the end of rows, etc. is where we stored them for each show. Daily, though, they gave us boxes and boxes of them. I'm sure they still do. Now sure how much money it costs the theatre's, though.
It is sort of shameful, though, if people profit on eBay from the extra copies as, technically, it is stealing for profit and most likely illegal. Of course, Broadway sells bunches of them at the "Broadway Flea Market" and donate that money to a great cause -- Broadway Cares/Equity Fights AIDS
Love and Stage Lights,
Angela Theresa