>>>why do these companies make available free version of their software to the masses ?
Look at the way AVG and Avast were "promoted" here, you cant buy advertising like that, and they get it for nothing.
As has been said, once you have the "free" one they then hope to sell you the full "internet security suite".
So the free software is a sort of loss-leader to get their name known.
Also if you have THEIR product on your PC you dont have one of their competitors.