It's under the same principle that Megaupload was shut down by the US, despite the company was not connected to the US or operating from the US.
This is why some websites block visitors from France (e.g. newspapers not willing to bother with GDPR for example) or that some sites block visitors, registrations and payments from the US (e.g. investment websites).
Here Telegram refuses to respect French law. Whether they are ethically right or not, it's one debate, but in the meantime, the detectives want to gather elements.
It's the standard way that police talks there. They don't send lawyers. Instead they put you in custody for interrogation.
A bit rude, I recognize it, though relatively common. Even the French CEO of Uber was sent in custody there.