In fact they're thinking like any company would if they sold a boxed copy of a game. Once you buy a boxed copy you cannot go back to the store two months later and ask to change for the other version.
Is it a bad line of thought? I do think so, the price that frictional has to pay to allow us to have access to the game on another platform is just bandwidth and no matter which OS I decide to install the game on (because I have a Windows partition), I'll use the same bandwidth to re-download the game...
What I find funny is that through the humble indie bundle I can download Overture again... but for OSX this time