That's true, but in general humans are concerned about the consequences of their actions and are metacognitive about them, or at the very least can be punished for making the wrong choices, its an iterative corrective process.

How do you apply the same logic to a neural net? It has no comprehension of desirable outcomes, or what its choices actually entail and never can.

There is no way to tell a neural net that a human is not a signpost directly, so it knows in the future no humans are signposts. The only thing you can do is train it on a larger data set and hope it "gets it".

You can tell a human being that a human is not a signpost once, and they can abstract it to all situations.

Hell, you can't even tell them that a red octagon on a post with the letters S T O P on it is a stop sign.