This is very unexpected and worryingly dystopian. Who decides what's illegal? The government, not Apple. Apple happens to start with child pornography because it's something everyone agrees is bad, and the moment you say anything against this system, you're labeled a pedophile so it's very hard to argue against it.
But what will happen in other countries with less agreeable laws? In Russia, gay pornography may soon be illegal. In China, all forms of pornography is illegal. In Japan, showing genitals is illegal. In China, showing pictures of Winnie the Pooh is illegal, and anything deemed "anti government" may result in a visit from their fleet of mobile execution busses that murder you and then distribute your organs to those who didn't say anything against the government yet.
It may start with child porn but this is literally a tiny step away from Big Brother, with governments being able to instantly see who is a potential "enemy of the state", flag them up and have them oppressed, deported or executed.
Ever taken a nude photo of yourself in the mirror at the age of 17? Congratulations. You're now officially in possession of child porn. Ever viewed porn, and didn't realize that the person in the photo/video is underage despite not looking underage, and not thought much about it? Well now that's a 20 year prison sentence for you. Ever heard of devices and accounts getting hacked? Well now someone can strategically upload child porn to your iCloud account to get you automatically incriminated to make you disappear for a few years. Great for oppressive governments that want to get rid of their anti-government activists. Ever wanted to win an election against a stronger opponent? Time to put some child porn on their iCloud account. Now they're automatically a sex offender and whatever they say it sounds terrible for them.
Just think of the possibilities, beyond the marketing.