I understand what you're saying skydriller but others can't know people are diabetic if their diabetes is being managed properly. If they travel regularly diabetics will come into contact with may people who have no experience, knowledge or understanding of diabetes. I for one only know what I've been told to allow me to do my job.
I think this rule of needing a letter is more to make sure people have the syringes for legitimate reasons and not for some illegal substance. Maybe?
If it was for security reasons as has already been pointed out, how do they suddenly become safe because the passengers have a doctors letter.
I still think we need to have a more robust system in place than a note from the doc.