A child is full of innocence. When they offer something, it is because they want to please. Therefore, in the eyes of an adult, it can disappoint and maybe stop any future offers, but for a child, it can be much more hurtful.
If not for the initial rejection then their future offerings may completely stop. They can see it as being wrong and so would not want to feel that shame again.
My youngest son is so generous with snacks and even giving me pieces of chicken. I sometimes feel guilty to take it, but you have to remember that they get a fulfilling reward from their offering.
Just remember that later you should also give him or her something too.