You make one major oversight. People, or at least myself, want zero tolerance for actually valid and reasonable rules that happen to coincide with laws. The school has no business telling any kid what they may or may not eat. They can choose what they make available for purchase, but that's it. Can they tell me what brand of mustards are allowed? Its about as meaningful as this 'policy'
Zero tolerance is for things like, violence, gun possesion, possesion of drugs, harassment, cheating, etc, etc.
This isn't a case of people not wanting zero-tolerance its a case of a couple morons trying to legislate what they deem to be proper behavior through policy through the guise of "We are looking out for the children". You are not looking out for the children, you are taking a knee jerk reaction of a moron. (aside what is non zero tolerance on this, you either have a forbidden item or you don't)
What next, outlawing non-mechanical pencils because of splinters and accidents with sharpeners? Telling me which brand of notebook I have to buy?
I love when a principal says something as apathetic as "whether or not I agree with the guidelines". If you are principal and you don't agree with the guidelines you should be taking steps to change them! Either state that you agree with them, or state that you disagree and are or have tried to take steps to make a change in the policy. If you don't have an opinion as the principal who the heck does? Aren't you supposed to be looking out for the well being of your students?
Does this excuse the third grader? In my opinion absolutely, in less you can demonstrate that they clearly understood that having that candy would land him detention (really detention in 3rd grade for that? you really don't have any larger problems in your school to solve?) Are the parents of candy providers to blaim? Yes, they should have known the policy, and if they disagreed they should get together with other parents and file a complaint.