The only PC sales figures they have are those made in brick and mortar stores by people who purchased physical copies. And if you're going to buy a Steam game, why would you go to the store and do it?
Because your download speed blows? I certainly don't bother with buying digital if I can get it on disc..... downloading anything more than a gig or two isn't something I casually think about doing. (obviously, there are times I buy digital - like the super Steam sales, or indie games that only come that way.)
edit: oh, and another reason is if you do lots of reinstalls. Like, if you have a small hard drive. Or I generally uninstalled and reinstalled Oblivion and FO3 whenever I wanted to try a new mod setup - better to start from a clean slate, instead of installing mods over old ones.
The anti-steam sentiment comes from a vocal minority of people who got VAC banned because of cheating, hacking, credit card fraud, griefing or similiar. In which case, I'm glad it is on steam

I don't play multiplayer or pvp. Don't think I've ever run anything that's got VAC involved. I think Steam's great as a digital store and indie-game distribution source. But I still think that requiring it (or any online, other than maybe a one-time check on install) for a single-player, offline, bought-on-disc game is absurd. And you couldn't
pay me to link games that I have that are free of that stuff to my Steam library - why would I want to cripple them like that?
(I've truly never understood the whole "oh, but now I don't have to put the disc in the drive anymore! And if I lose or damage my disc I'm safe!" comments. Putting a disc in a drive has never been a burden. And I'm always surprised when people say that they lose or damage discs - seriously, how do you even do that?)