By "rule" I was referring to the criteria by which exceptions were granted. I'll grant that wasn't clear in my language, but that's what I was driving at.
Because [the] POTUS has a national security need for immediate communications because they're the head of the military, have access to the football, and may need to make key strategic decisions).
The Secretary of State's job is also quite important, you know. They're also pretty high up in the chain-of-succession for direct access to the same duties as the POTUS.
Or they received an answer but either didn't understand the answer or just didn't like it. ... I'm sure the NSA gave them a full report on why cellphones are risky and how they can be compromised. But I don't know if that information was received by state or understood.
Drawing from the trove of recently released emails, the article itself says that most likely they were informed of (and understood -- or at least her staff understood -- the risks of) using private cellphones ("her staff was fully aware of the security risks associated with using her BlackBerry.") What they didn't understand is why a modified cellphone was considered a secure enough solution both for the POTUS, and for the predecessor SoS -- but not for the current one.
As to whether Clinton herself really understood the risks, or she did, why she continued to use a non-secured device, anyway?
That I don't know -- and I won't offer any defense for, either.