Google’s structured data guidelines describe what you should/shouldn’t do for getting one of their search result features, not what you should/shouldn’t do with structured data on your pages.
Google, as only one of possibly many consumers, doesn’t (and shouldn’t) decide what kind of structured data is useful. It’s perfectly fine to go against their guidelines, the only "risk" being that you don’t get one of their search result features.
In your first example, the page is giving an
AggregateRating for a
Thing named "Cufflinks". Their structured data is not really expressive, but the text on the page explains that this is the average customer rating for all of their products in that cufflinks category. The structured data doesn’t seem to be misleading (it’s vague, however), so I don’t see a problem with this. However, Google would ideally not show an AggregateRating rich result for them, based on their guidelines, as this is clearly a category page and not about a specific product/service:
Refer clearly to a specific product or service.
Provide review and/or rating information about a specific item, not about a category or a list of items.
(Note that you quoted the guidelines for product rich results, but the page in question isn’t using
Product structured data.)
In your second example, the page is giving an
AggregateRating for a
Product named "car insurance". From a quick look, the page doesn’t seem to be about a category/list; while this is also some kind of category/entry page, the focus seems to be on the car insurance. And the structured data conveys that the ratings are only for the car insurance, not the other insurances. So it seems to be fine that Google displays an AggregateRating rich result for this page.
Receiving stars for category pages
Is there an ethical way of receiving stars for category pages?
According to Google’s current guidelines, no.
But that shouldn’t stop you from providing that structured data on category pages, if you think it can be useful for a consumer.
If you don’t want to appear to be misleading (in the hope that Google doesn’t detect that your page is a category page), be expressive in your structured data, e.g., by using a
CollectionPage instead of an
ItemPage. Give consumers as much information as possible about what your aggregated ratings are about, so that they can better understand if it meets their own guidelines or not.