Earlier quoted context omitted.
If you know your services are going to be used to commit a crime, then yes, that makes you an accessory and basically all jurisdictions (I know basically nothing about French criminal law) can prosecute you for that. Crime is, y'know, illegal.
I'm appalled that you would argue in good faith that a tool for communicating in secret can be reasonably described as a service used to commit a crime. Why aren't all gun manufacturers in jail then? They must know a percentage of their products are going to be used to commit crimes. A much larger percentage than those using Telegram to commit one.
That's because you're assuming facts not in evidence and painting the broadest possible argument. Obviously we don't know the details yet, but it's not unlikely that this situation was a bit more specific.
Consider:
F: "We want you to give us the chat logs of this terrorist"
T: "OK!"
F: "Now we need you to give us the logs from this CSAM ring"
T: "No! That's a violation of their free speech rights!"
You can't put your own moral compass in place of the law, basically. That final statement is very reasonably interpreted as obstruction or conspiracy, where a blanket refusal would not be.