Well, as +Casey Barker
points out, many employers offer explicit awards for filing an "invention disclosure", often from hundreds up to a thousand dollars. For many employees, this bonus provides a real incentive to write up even the lamest ideas. I think "patent hamster wheel" is the derogatory term: 'leap, run, leap, ooh I got an idea, stop, write, run leap run...'
As +Ryan Barrett
points out, prior art is not investigated. I was particularly pissed off when a "master inventor" (>25 patents) described a particularly juicy one he got recently, double-bonus, as it was deemed a core technology: I had implemented the same thing in an open-source project some years earlier, in GPL'ed code. He's a nice guy, but when we talked about it, he brushed it off -- what's the point, call the patent attorneys, the management, and say "oh never mind"?
Patents are worded so that they have a long list of claims, sorted from very broad claims to very narrow ones. So even if my prior open-source work invalidates some of the broad claims, well, maybe some of the narrowest claims still stand. Pointless to investigate prior-art, until you absolutely need to, in court. Its really really hard work to investigate prior art. much
harder, by orders of magnitude, than coming up with the original idea.