Like you said, for attribution purposes the NSA had to get its keys elsewhere. I'm asserting that it's not just attribution that's on their mind.
You can't forge certificates without associating them with some specific CA.
If you forge a certificate for a pinned site, you risk detection.
If HPKP is widely deployed, every site could have that risk.
Unless you've popped all the CAs, the browser vendors can respond to detected forged certificates by curtailing the compromised CA. Meaning the NSA has to compromise another CA to continue their activities.
There aren't unlimited CAs to work with.
Stipulate that NSA doesn't care if attacks are attributed to them. Certificate surveillance is still an operational problem for them.
aside from the economic cost of "burning" CAs, attribution itself seems to be a consideration, e.g. in other contexts like TAWDRYYARD or LOUDAUTO where "All components are Commercial Off-the-Shelf (COTS) and so are non-attributable to NSA."
The idea that a major government malware contracting effort was required to pop a particular Hungarian CA (presumably for deniability reasons) tells you something. The USG virtually undoubtedly controls several RSA keys that can be used to sign arbitrary SSL/TLS certificates. Why didn't they just use one of those?
I assume it's because they're expensive, and every time you use them, you risk burning the CA they're associated with: the major browser and OS vendors will excise your root keys, or attach constraints to their use.
Kim Zetter, for understandable narrative reasons, uses Gmail as an example of the kind of site that a CA-hijacker could compromise. But Gmail is the dumbest possible site to target with a traceable compromised CA key, because it's key identities are pinned in Firefox and Chrome; if the key indicated over the wire disagrees with the browser binary, the browser flips out.
This is why HPKP, TACK, and similar pinning/continuity/attestation frameworks are such a good idea. Over the medium term, they allow the users of the Internet to surveil SSL/TLS keys and detect compromised CAs.