Ok, I am wondering if I am missing something really obvious here.
Lots of bikes have tubular aluminum racks on them. My racks have nothing specific to attach a tail light. All tail lights come with a seat post clamp, and a clip for attaching to clothing or bags. Only two tail lights I have found specifically mention that they attach to a rack: A topeak tail light on a topeak rack with a tail light mount, and a blackburn mars 2 light that says it can attach to a blackburn rack.
No description or picture is given for the blackburn Mars 2 light attachment, other than "no tools required."
Now, I would think that your rear rack would be one of the first places many people would want to install a tail light. Then things on top of your rack will not obstruct the tail light. Yet no info is given about what lights can do this, and how they do it. Does everyone just chuck the seat post mount, head to the hardware store, and rig their own method?
I have three or four blinkie tail lights, none of them attach to a rack. What gives?