They 100 percent sit in Russia, which will 100 percent ignore this, even if their identity gets uncovered. So it's perfectly safe to continue for the operators.
> The Member acknowledges and agrees that the registration of Internet Number Resources does not constitute property and the registration of Internet Number Resources in the name of the Member or a third party does not confer upon the Member or the third party any rights of ownership. The Member acknowledges that any Internet Number Resources deregistered by the RIPE NCC may be re-registered to another party according to the RIPE Policies.
If whatever service provider in Russia won't shut off their site, I imagine that the next step would be getting a court order in the Netherlands to revoke that provider's IP range.
You get censorship resistance and it also doesn't leave a trail that leads to your location or requires payment methods. All of which leads to deanonymization.
The main way that an adversary would identify the location of an onion site would be to shut off the power/internet in various locations. That would be an unlikely step against some book piracy, imo.