At the time we posted Whatever Happened To Due Process we were unaware that we were just one of many registrars receiving these notices from the London (UK) Police.
We have since been made aware that this was part of a larger initiative against the BitTorrent space as a whole, and that most if not all of the other registrars in receipt of the same email as us folded rather quickly and acquiesced to the shakedown orders.
Since there were no charges against any of the domains and no court orders, it may be at the registrars' discretion to play ball with these ridiculous demands. However – what they clearly cannot do now, is prevent any of those domain holders from simply transferring out their names to more clueful, less wimpy registrars.
If any of those registrars denied the ability to do that, then they would be in clear violation of the ICANN Inter-Registrars Transfer Policy.
Section 3, Obligations of The Registrar of Record clearly spells out the reasons why a registrar may deny a transfer-out request, and they are limited specifically to cases of fraud (the domain was paid for fraudulently), a UDRP proceeding or, hey, get this one "Court order by a court of competent jurisdiction", as well as some administrative reasons (like the domain was registered less than 60 days ago).
What is conspicuously absent from the list of reasons why a registrar that actually complied with this lunacy can now deny a transfer-out request is "because some guy sent you an email telling you to lock it down".
Any registrar that has taken one of these sites offline that now impedes the registrants of those domains from simply getting their domain names out of there and back online somewhere else will then be subject to the TDRP – Transfer Dispute Resolution Policy and if they lose (which they will) they will be subject to TDRP fees assesed by the registry operator, and to quote the TDRP itself "Transfer dispute resolution fees can be substantial".
This is why it is never a good idea to just react to pressure in the face of obnoxious bluster – in the very act of trying to diffuse any perceived culpability you end up opening yourself to real liability.
Even when Verisign seizes a domain out from under you (like they did one of ours earlier this year ) it at least happens under a "sealed warrant" – blessing it with a veneer of Soviet-Era style legality.