According to The Post's report, roughly 2% of the top 1,000 grossing apps (i.e. the apps that earn the most money) are cons. It may not sound like a lot, but that means there are about 20 malicious apps popular enough to hit the top 1,000. These scams have apparently cost users an estimated $48 million, with cons that range from fake user reviews to apps that trick you into paying for services your device already uses.
Apple claims to typically find and remove apps like these within a month or so of release, though it's clear some still fall through the cracks. These claims of following strict guidelines may also give users a false sense of security, resulting in less scrutiny for potential purchases. If users believe the App Store is safe, why should they suspect something that looks official?