Hi Brian,
I understand that you are really annoyed at the situation. In an ideal world life and its experiences would be fair. It was underhand to not tell you until the end of the day, but you really have no rights whilst on a probationary period. All they need to say is that you are unsuitable for the role, end of story. The manager may have had second thoughts as to having another diabetic on their team and decided that it would affect their performance as a whole. Not very ethical I know. Take this as a learning experience and be prepared next time. Check your sugar level regularly when on a new job to see if any adjustment is required. An employer should let you do that. Yes, you should be allowed to monitor your condition, it is your responsibility to manage it as well as doing your job. You are getting the same pay as a non diabetic in the same role. But again you should be allowed reasonable time for that, in an ideal world. I know this all sounds harsh, but I speak from experience. Be savvy and prepared.
Good luck with the next job.
Pompeykeith