It wasn't one big event that someone got arrested for using the app. It was, civil polices were arresting anyone who uses crypto-message apps like ByLock.
We've seen cases where people sent to prison because they are wearing certain T-short. Really.
edit links:
https://www.theguardian.com/world/2017/sep/11/turks-detained...
https://www.huffingtonpost.com/mahir-zeynalov/turkish-police...