My guess would be that iCloud UX is designed around a lifestyle with a single account per person, and they want their staff's UX patterns to mirror their customers' as closely as possible.

When you wake up in bed you don't grab two phones, you grab one.

One of the account recovery methods is to request that Apple email you an "unlock my iCloud account" web link.

If they were to send that email to your locked iCloud account, you wouldn't be able to receive it.

Operating by fiat? Because they can.