I simply don't buy something when it is an unknown. And a free weekend costs them nothing. But if the game is a year old, I won't even care about it even if it's free.
In fact the free weekend should have been EXACTLY on the first 3 weeks after release, because that is when most people care about the game and would want to give it a try before buying. That has nothing to do with greed (specially since a free weekend does not cost them any sales at all)