So, this is because it's an older game? Or are the going to be getting into the virtual goods business?
I would say it's a move to get new Steam users, particularly users with Macs.
I imagine Portal downloads and purchases have also probably reached all-time lows. With the press coverage and meme-ification it got, there are probably very very few people left who would be willing to buy it, and have not because they haven't heard of it yet. Thus, by giving it away free to people who wouldn't have bought it anyway, no money lost.