I have played with faucets in the past.  Haven't run into any scams yet.  Then again, I have never needed to download anything before. 

Most work on a simple principle.  Which is that it's not worth your time and effort.  Places like Google ads may pay $0.005 for every view of an ad on a webpage (I don't know the exact number).  And the owners of the faucet page are giving you a small part of that like $0.001.  So you watch a few, and earn a few satoshis ect and think, that is pretty cool, you are making money for doing nothing.  Until you notice you dedicated 2-3 hours of your time, and managed to earn $0.02 for your efforts.