As others have mentioned, context matters a lot. The arrested group came back from Syria (where they fought alongside YPG against ISIS) radicalised, and were monitored ever since then. Their alleged crime isn't using Signal, it's just that a French anti-terrorism law allows to be arrested for "organisation with the intent to commit terrorist acts", which the DGSI(internal intelligence services) claims a radicalised group of people calling for a revolution, using encrypted communications, having a bunch of hunting weapons and ammunition, and materials for explosives is. A big stretch on the surface, but they were monitoring them for years, so who the hell knows what else they have.
The real problem is the unlawful detention of one of the men, for which a court finally intervened and he has been freed under surveillance.
The real problem is the unlawful detention of one of the men, for which a court finally intervened and he has been freed under surveillance.