Wasn't this locked?
Anyway, I foudn this:
LINK
Valve announced today in their official forums that they eliminated roughly 20,000 Steam accounts yesterday which were found to be attempting to pirate Half-Life 2 in a variety of ways. Defending this rather aggressive anti-piracy decision, Valve adamantly stated in their post that the method by which they were able to trace and catch would-be pirates was a simple procedure, which should eliminate any ambiguity surrounding this mass culling of Steam users.
Similarly, Valve also commented on rumors alleging "fake key or fake warez or hack instructions to trap people" into pirating the game only to find it was a anti-piracy ploy to prevent and perhaps even sue pirates. According to Valve, the hack came from the "community" as one might expect, and the company stated that Steam the accounts that were closed were not related to this untrue rumor, but instead that reasons for account cancellation "includes Credit Card fraud, theft of accounts you do not own and using cracked versions of Valve games."
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Piracy is illegal, "not a good thing" and people who are willing to commit credit card fraud to get a game are just bizarre, but...
I'm a bit concerned about the last reason for deleting accounts; "using cracked versions of Valve games". Does that include the good ol' No-CD patch?