As PPs are pointing out, similar to when your student's parents forgot to send her with a sack lunch on the zoo trip day, there are a myriad of reasons why these things can be easily missed by families trying to get kids off to school. Getting so worked up and accusatory about what is really a simple mistake (yes, I do realize said mistake can be very dangerous for the child with the allergy) is not productive.
And I agree that is is a massive overreaction for the teacher to jump to her buying snacks for the entire class every day all year based on one incident early in the year of one kid forgetting the rule. That is a huge amount of liability and cost for that teacher to take on personally. Not to mention, as a parent, I might get pretty annoyed if the snacks provided were not tings my child would eat or were too processed and not as healthy as I would pack, etc but you told me we could no longer pack our own and there was a snack time. I am really surprised that a teacher would take on all of that based off of one incident.