I think it's completely reasonable to go to the principal about this. It's not just about your kid, it's about our entire society. Obesity is a major threat to public health, and the health problems associated with it are affecting kids RIGHT NOW, TODAY.
Candy and treats aren't evil, and in the old days something like 1 candy cane per kid would have been the norm- not three bags of candy. Not tons of cupcakes filled with high fructose corn syrup. And on top of the treats, the kids weren't being fed a bunch of junk food at lunch.
It may be your responsibility to make sure your kid eats well, but with the growing health crisis facing this country, with 1 out of 3 children born today destined to become diabetic, it's everyone's responsibility to promote a health conscious environment in the institutions we send our children to for hours a day.
At my daughters' school we don't allow candy or sugary snacks. Sometimes they sneak through, but since the policy was started school-wide we definitely have seen a major decrease in the junk. They have also gotten rid of the chocolate milk at lunch, which contained as much high fructose corn syrup as a can of soda. We have an organic garden that the children care for, a wood-fired pizza oven that the kids make healthy pizzas in on special occasions, and I have done several healthy veggie cooking demos at lunch times, using equipment and organic produce donated by local organizations and businesses. We have also hosted parent information sessions about healthy eating, and had the famous Chef Ann, The Renegade Lunch Lady (www.chefann.com) come to our school to promote healthy school lunches. This is at a public charter school, and it was all made possible by concerned parents like you, getting together, talking to the administration, and getting involved. Try not to get too angry, look at it as an opportunity to do something positive for your school. I'm sure you will be able to find like-minded parents, and you can take a bunch of statistics to back up your argument to the administration.