A couple of fallacies here.
> considering the start of this a while ago was the fear that Apple would scan your entire photo library without consent, I feel like this is fair trade.
The first is that since they were going to implement a much more intrusive approach but halted due to incredible backlash from users and the tech community it’s OK that they now implement a much less intrusive approach.
> It makes sense that the quite literal public link, open to anyone, is also accessible to Apple.
The second is that a publicly accessible link is very much not the same as “open to anyone”, the link itself is the knowledge gate required to access the data.
Let’s say I use iCloud and need to share my tax documents with my accountant who doesn’t use iPhone/Mac. I turn on public link sharing temporarily with the knowledge that only people who have the link could access it, thereby still being secure so long as neither party is careless in their distribution of the link.
My expectation as a user is that only those with the exact link can view my tax documents. But as it stands, Apple is now using their privileged position as iCloud administrator to unencrypt my file and scan it simply because the link is stored in their database.
Publicly accessible != publicly distributed
The alternative is to only share a link with individuals you grant access to.
> When you share content that’s stored in iCloud, such as a photo in iCloud Photos or a file in iCloud Drive, you can choose to Share a Link or Send a Copy of the item. If you choose Share a Link, you can either share the item with “People you choose” — only people you invite or approve — or with “Anyone with the link," which makes it accessible to anyone who has the link, including Apple.
> My expectation as a user is that only those with the exact link can view my tax documents
Security through obscurity has never worked. Access control lists do.
> Let’s say I use iCloud and need to share my tax documents with my accountant who doesn’t use iPhone/Mac.
iCloud photo albums can be shared to an email address - the recipient does not need to have an account with Apple. I don't know offhand what the user experience is on the receiving end, but there's a clearly signposted way of doing this, and it doesn't require the album to be set as public.
The fact that Apple doesn't see the link alone as the "knowledge gate" or "access restriction" says it all.
It’s pretty stupid to say that you publicly share a document, accessible to anyone, but that you hope that a single person will be able to access it.
If you sent that link through WhatsApp, they access it to generate a preview (and who knows what else?). Same for any number of other services.
iCloud, like all other file distribution services, make absolutely no distinction between publicly accessible and publicly distributed. Not should they. They are distributing it… publicly.
… because you made the file public.