Once again, I didn't get the memo

The one banning turn signal usage, that is.
Look, it's not just a matter of legality and safety.  If I'm sitting at a stop sign waiting to turn left out of a side street, and you turn right into the side street, I'm going to be annoyed with you for not signaling, so I waited longer than I needed to.
Similarly, if you're waiting to tun left out of a side street but don't signal, I may assume you're waiting for traffic to clear to turn right, and not leave space for you between me and the other cars waiting for the light.  So there's self-interest, too.
Plus, of course, safety.