In my opinion this is a rather silly feature.
As a respondent I do not want to be bothered each time after I decided not to answer a question. After the third time I close the browser.
As a researcher I want to get an answer. We mostly ask for opinions, ratings, etc. which is always answerable.
So all questions are mandatory, sometimes with the option "WN/DK".
You are right: In mandatory questions the missing item is displayed in red.
In these "soft" question it is not - it is not an error.
You only see the normal question after you click "Yes, I am going to answer"
When having item-non-response, it is hard for respondents to see which item they left out.
And this shouldn't be a big issue, if you stay to the recommendation "One question per page"
And it should go without saying that one does not bombard a participant with an array that exceeds the page height so that he has to scroll.
This should be splitted into several pages.
Well, this is my opinion.
Joffm