The thing is, they make the most money on the platform they take the most money per sale from. That is always going to be Linux, because in the long run, Apple and Microsoft can lock down OSX / Windows and the consoles are already taking per sale cuts for MS / Sony. Valve can only bet on desktop Linux (aka, not Android) to never stop arbitrary application installation.
Well, no, they're not currently paying any licensing fees on Macintosh or Windows systems. So they make the same profit per sale for all three platforms. Now, this may not remain true forever - but if HL3 comes out before the status quo changes, pure profit wouldn't be the incentive (and in any case, in the current steam model you can buy stuff for Linux then run it on Windows later anyway). You could argue for driving people to Linux by giving them perks, of course - but that has nothing to do with present-day marginal costs.