Researchers take down Koobface servers

The only thing I don't get about events like this where a botnet is stopped is that if the security researchers have caught it and also can get control of the servers through law enforcement, why don't they turn them on itself and tell the infected machine to download a program which clears the infection?

Is it too complicated, or perhaps perversely against the law?

The other one is like the recent worm that was peer-to-peering the commands. Why can't they just take it over?

"They also messed up their victims' search results to trick them into clicking on online ads, generating referral money from advertising companies. More cash came from fake antivirus software that Koobface can sneak onto victims' PCs. "

surrealism.