Earlier quoted context omitted.
Because some things like terrorism and child sex abuse are harms to society as a whole, and even private individuals have an obligation to help combat them. Durov has a service where by design it's hard to filter out that kind of activity, and he's effectively (if not explicitly) helping protect that activity.
So is France going to arrest the owners of HP, because their printers can't filter out CSAM?
A more comparable example, is France going to arrest someone who maintains a printer in an office and knows an employee is printing CSAM but doing nothing about it?
I hope they would, this is the boat Telegram is in.