Thanks for the feedback and suggestion, Chris,
I don't regard that as a bug, exactly, because the Default Mark for a
question is merely a default and can be overridden when the question is
in the quiz. If the penalty regime were hidden when the default mark is
zero you could set up a question that was worth marks and had a hefty
penalty regime that was hidden from students. I also don't wish to hide the Penalty Regime when it's zero because students need to be explicitly made aware of the fact it's zero in a normal quizzes, where most questions have a non-zero penalty regime.
Certainly I think it would be good to hide the penalty regime if the actual mark for a question in a particular quiz is zero, but I don't know how to get at that actual mark. Perhaps Tim Hunt can advise if there is a way to do so? That would definitely be the best fix, if it's possible.
I'm reluctant to add further complication to the authoring UI, which is already overwhelming for many users. However, if students are being misled by seeing Penalty Regime: 0%, an easy tweak might be to display instead:
Penalties do not apply to this question
whenever the penalty regime is zero. Would your colleagues be happy with that? [I'm not totally sure that I'm happy with it, but let's see what people think first.]
By the way: if your colleagues are setting ungraded questions, are they aware of the Python3 Sandpit question type? It doesn't solve this particular issue but does lessen the confusion with graded questions by displaying results in a very different format, without ticks and crosses.