TheOneGuy said:
I don't see how it points to anything other than "hey there aren't a lot of third-party games on this list!"
Precisely the reason why. Half the 'third-party' games on that list are Valve-published. As far as I can tell, the few others released on Steam at the same time as retail.
My guess would be that Valve has a number of requirements that must be met. Probably something about the CD-keys to ensure compatibility with the current Steam validation system and to make sure people can't generate valid keys to get free copies from Steam.
That would certainly explain why few publishers would be willing to do this, and why it would be nearly impossible to add the ability after the retail release occurred. The only exception would be the original Half-Life, because the system was built specifically around that game.
Those sorts of requirements would make sense and would explain why Valve-published games always do it, while few others can. Unfortunately, Valve has never mentioned what requirements they have.
Common sense tells us they would not accept some. For example, if the code to unlock a game was a single digit, I can't imagine Valve would allow it. But what if it was only 10? They must have some sort of policy governing what they accept and what they reject.