YOU FOUND A METAL BOTTLECAP!!! I've been looking all over for one of those . . . along with a piece of broken glass, non-descript rock and an old air freshener!
On a serious note . . . ah, the great question . . . why would anyone feel compelled to drop in some of these questionable items??? Who knows? I don't know if people just feel compelled to put something, anything in the cache if and when they take anything (because I honestly don't think most folks consider dropping in a rock they found along the trail as an equal trade for a $1 bill . . . and if they do consider that a good trade I have several pieces of junk . . . I mean valuables . . . hanging around my house that I would love to sell to them at a very, very special deal )
All I can say is that I try to maintain my caches and when I spy junk I pull it out . . . and for my part I try to leave things in caches (regardless of whether I trade or not) that I think others might like . . . or at least would not view as junk. Items in clean baggies or self-enclosed wrapping are always nice . . . I mean finding a new set of corn cob holders is great . . . but finding those same corn cob holders rusting away at the bottom of a cache and not knowing if those were once used by someone is not so great.
In general I like to see tradeables that are either in their original wrapping or in a quality Ziplock-type plastic bag . . . if an item is used it should be clean and in good condition. To me even clean items that are just placed in a cache can get dirty pretty quickly . . . which is why the books, stuffed animals and even pencils with eraser tops that I sometimes place in caches are typically in plastic bags.
I guess what I'm trying to say is that I feel one of the best ways to keep geocaches from being geo-junk is by trying to do the right thing and set an example . . . if folks see clean caches with items that are packaged and there is no junk they will come to believe that this is the way a cache should be . . . but if they see caches that are dirty, full of junk and have mildewing stuffed animals then they will come to believe that this is the way a typical cache should be.