Hi,
Your right, you shouldn't exercise if your BG's are over 13mmol ESPECIALLY if you have ketones present..
I was told this is because..
If your BG is 13mmol it could mean you aren't getting enough insulin..
If your not getting enough insulin your body breaks down fat cells for energy and produces ketones...
If you exercise on top of this your body needs more energy due to the increase in activity..
But again with not enough insulin, your body will again break down more fat to fuel the body producing even more ketones..
And ketones + Ketones = a very poorly person!
Hope this explains simply for you