You know I don't think "they should go to a second-party developer website" counts as a valid argument. They (Guerilla, Insomniac, etc.) work directly with Sony, of course they're going to know how to "properly" develop for it. And people seem to be forgetting the amazingly horrendous rash of badly ported PS3 games early in the console's life as a result of being difficult to develop for. This isn't a "valve" problem, this is an industry-wide problem. Valve's the only one speaking about it.
Jumping on Valve's back for refusing to develop for the PS3 is like getting on Square's case for developing solely for the Playstation and its descendants for almost a decade. It's like going after Rare for jumping ship from Nintendo to Microsoft just on that one principle alone (nevermind the fact they haven't been doing so well game dev wise since that move). It's like getting pissed off at Epic Games for not making Gears of War 2 for the PC, though actually this one is probably closest to the Valve situation in that they had someone repeatedly giving reasons why not. Although I think Valve's reasons are slightly better than Cliffy B's "dur-hurr piracy" reasons, but I suspect money is the central issue there.
In any event I'm reiterating a point that Valve has already tried their hand at PS3 development, it's not like they didn't try. They put out a version of The Orange Box for the PS3, a version, I might add, that has been rated less than its PC and Xbox 360 counterparts. I have no sales figures, but, say for example that the PS3 version did indeed not sell as well as the other two. Doesn't it make more sense from a business standpoint to not spend resources on PS3 development if you aren't getting as good a return on it? Again that is just speculation, I do not have any hard facts about it unfortunately.