IRC requires a dedicated TCP connection, whereas Usenet can use uh... very creative methods of sharing news. TCP reconnects if you ever go offline, so it's always reporting when you come online. Even more if you're using "Keepalive" packets, and IRC has a PING/PONG message on top of that. IRC is very low latency and anti-federation, so you cannot schedule to come online later to deliver your messages when you're not there. Nor can you read messages coming in when you're offline.
And by "broadcasting" I mean IRC servers generally will show you as present in the room to everyone in the room, only when you make a TCP connection to the server. You're not just informing the server, it's broadcasting that info to other clients.
And uh... TLS doesn't help with any of that.
