Some of them got used to shoot down our own planes, no?

I've never heard of that happening

We don't always know what shot down a given aircraft, and one black-market MANPADS is much like another for most purposes, but from a search around at least one Chinook shot down in Afghanistan was thought to be by a Stinger left over from those days. Also spending a fair bit on a buyback program seems like an indication of regret.