remember that they took the relay because a *victim* contacted it, not because they think the "guyz behind the software" did.
Civil sue them for stupid thinking / false arrest confiscation, loss of service and use, public tarnishment, bad training, etc.
what can be interesting for police by unpluging those guards relays ?
Nothing. Well, off topic, unless they were researching confirmation or partitioning attacks.
Typically that's why cops choose not to bother Tor relays -- because they know there will be nothing useful. That's actually why the torservers.net people suggest *not* using disk encryption. Having no barriers makes it much easier for the police to realize that there's nothing useful to them.
This falling over may perhaps not be preferred by operators who like to create wins in the crypto war. You want police to go get their warrants, waste their time and money, just to prove nothing upon decrypt... then you have higher recorded, thus marketable, percent of nothing found among all forced decrypt cases. Instead of closer to 100% of such cases just confirming already forgone criminal cases. Having higher barriers and costs and demonstrably less fruit ratio can make such seizures more unlikely in first place.