There's an alternative that is more user-friendly which the browser provides, but only for app install interstitials. Since there's no built-in alternative for other kinds of interstitials, it makes sense that they're left alone (although the crappier your site, the less time people spend on it, and presumably the farther it falls down the search results).

Also, the blog post states they're essentially calling these sites mobile-unfriendly, which I think is entirely fair. All full-page interstitials are annoying, on the mobile web or desktop web, but only "install our app" interstitials are unique to mobile. So if you view "mobile friendly" as meaning "as good as desktop web", banning bad things that are only relevant to mobile makes sense.