I managed to figure out what's going on. write tries to use standard output to determine who the currently logged on user is. Traditional cron would have stdout captured by a spool running as a user process; however systemd cron would have it captured by a systemd process running as root.
While you cannot write as another user in any case, you could traditionally use write in a cron job as it spools to you. Now it doesn't work. It's really easy to knock this check out of write.c in bsdmainutils; however it's a bit harder to get it to do the right thing. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org