One can't exit from dbus-monitor by typing Ctrl-C. I can only guess dbus-monitor catches SIGINT but doesn't process it correctly. It will quit on ctrl-Z but generate a core dump. It's one of those minor annoyances if you see what I mean.
It's annoying yeah, as a frequent user of dbus-monitor I agree. But such cosmetic things needs to be filed upstream.
Fair enough. https://bugs.freedesktop.org/show_bug.cgi?id=13336