NTA
You are not the issue. You are encouraging your child to eat healthy and it sounds like he is generous with food... which is great. It shows he is learning really great food associations.
But I do think you need to tell Sam to stop sharing his snacks. Not because I don't think other kids should have fresh food, but because the potential for causing an allergic reaction is huge. The fact that a teacher has not caught on to this is a potential lawsuit/disaster waiting to happen.
I used to work at a daycare/kindergarten where we were told very clearly that kids are not built for survival at that age. They want something other people have and they don't know their allergies very well. Your kid wants to make friends and share, and it might cause a problem with a kid who doesn't know any better.
So I think, while it is great your son is so generous (along with yourself) it is obviously causing issues at the school.