And it's not true that with electronic communication you either get full privacy or none. You can have end-to-end encrypted messages with unencrypted metadata, so that when police observe a message implicating the sender in a crime (e.g. on an arrested suspect's phone) they can get a search warrant for the IP address or phone number associated with that account and then visit the owner in person to look at the messages on their phone. This doesn't allow police to read everyone's messages all the time undetected, but does allow them to read specific people's messages if they get a warrant.

Since Telegram doesn't only have unencrypted metadata but also plenty of unencrypted messages, there must've been many cases where a search warrant would've yielded lots of useful information. If Telegram didn't properly respond to all warrants, it seems fair to launch an investigation.