I can tell you that as a parent of a child with severe food allergies, I am thankful that the preschool was diligent about this.
I understand and sympathize with your frustration (and I'd be angry about being out the $100), but this could be a matter of LIFE or DEATH. I am not exaggerating. Please read this news story and know that my heart breaks for the family:
http://www.wtvr.com/news/wtvr-chesterfield-student-death-...
There is a problem with using a "pre-approved list of foods." And that problem is that while a food label may be listed as safe at one reading, the next time a person goes to buy the item, the manufacturer could have switched ingredients or manufacturing processes and now the food is not safe anymore, but is still erroneously listed on the "pre-approved list of foods." The best real-world answer is to always read the ingredients, and the warning labels; read the entire ingredients panel to search for allergens. I know it's a pain, but your taking an extra few minutes to read it could save the life of a child in your child's preschool.
Always, always, always ask the school (both to find out the policy and to specifically ask if there are any known food allergies for your child's classroom) before sending food treats for children. And instead of doing food items, consider doing other items like specialty erasers or pencils or temporary tattoos, or coloring pages with a theme, or sponsoring a craft activity for the day (everyone get's to make a crown or a magic wand), etc.
You can purchase products from Enjoy Life that are free of 8 of the major food allergens (free of wheat/gluten, dairy, peanuts, tree nuts, egg, soy, fish or shellfish). Here is their website: http://www.enjoylifefoods.com/our_lifestyle/allergy_frien....
I would also suggest contacting a nut-free bakery in your area. Just google "nut free bakery" and your location, and I'll bet you get a hit.
Arm yourself with information about food allergies (and how to avoid allergic reactions!), and realize that by doing so, you could very well save a child's life. No exaggeration. This is an excellent site for learning more information about food allergies: http://www.kidswithfoodallergies.org/.
My son goes to a nut free school, and parents are warned to not bring anything with nuts to the school, and the teachers are diligent to check labels.