The root cause comes from these twelve words:
Since only one primary account can be signed in at a time ...
Authentication systems that allow only a single signed-in account are defective by design. Approximately 100% of all people have more than one account that they need to be signed in with.
I have half a dozen Google accounts for specific purposes — one for work, one for personal, one for electronic music tablets to which multiple people share physical access, a couple of old domain-specific accounts that got converted from free work profiles, and I'm probably forgetting some. Not all of these are useful, but if I could not have continuous access to work and personal, it would be a tremendous pain in the backside.
And encouraging people to merge profiles like that is nuts. It all but guarantees leaks. A confidentiality agreement typically protects things you learned in the scope of your employment, but after you are employed, if they continue to send you confidential information, there's a decent chance that you aren't under any legal obligation to keep that secret.
This is just a really bad way to do things, driven by a fundamental flaw in their architecture. And it doesn't just cause problems for Apple. It causes problems for their customers as well. I can't have a separate iCloud account for my work. I have to use a Google account. And more than that, when I hit the maximum number of AirTag devices (which I'm about to reach), I can't add additional iCloud accounts. I'm hard-capped at a single account.
While we're at it, the limitation of a single user per iPad stems from the same defective design process. All of those sorts of 1:1 mapping decisions between accounts, users, and devices need to be fundamentally rethought, because they were always a mistake. Maybe now that Apple is being bitten by it, they'll finally do the things that users have been asking them to do for the last decade. Bonus points if they lose their lawsuit because of a lack of a modicum of care on their part, because that just might be enough to wake the sleeping giant and get them to fix this mess.