Unlike other chat services Telegram has a “social” aspect and search capabilities for locating public discussion channels.
Furthermore E2EE is optional and most people don’t turn it on and is certainly not on in public channels.
While techies are freaking out about an attack on encryption the articles I’ve read so far don’t mention anything about encryption or otherwise it seems that French police is concerned about moderation or attempts at moderation of those public channels, that Telegram specifically refuses to moderate.
Perhaps this will be an attack on encryption by stealth but at this point that’s not what it looks like.
As a personal anecdote when I installed Telegram a few years ago and searched for my city’s name the top 20 results where channels offering to sell you heroin - which I thought was so blatant as to be certain it was police sting operations - but who knows.