I never understood mechanically how a vending machine prevents people from accessing goods through the dispensing panel. From what I understand it's not a locked door that unlocks only upon payment, dispenses goods and then re-locks. In my experience it seems to be a non-automated mechanical system powered by the weight of the dispensed goods when they fall into it.
I remember as a young child I would out of curiosity try to stick my hand into the panel and see if I can reach up but was unable to do so as my hand would be blocked by what felt like a literal wall. So I know there is some mechanism in place to ensure goods can only flow out and nothing can come in, but I don't understood mechanically how such a thing works.