I'm not really familiar with the details, but it's my understanding the Seattle PD are under some sort of probation system with the DOJ, which I'm guessing the FBI was involved with. So I think this sort of thing does happen, though I guess not enough.
They have a certain amount of ability to do that if the state or local government is violating Federal law. But they do not have general oversight or police powers, since the Federal government as a whole does not. Nor is it supposed to.
Aside from activating / deactivating the camera (with clear guidelines and penalties for doing / not doing so), and connecting it to a network at the end of a shift, it should be a black box. None of the IT functions should run out of the same police force.
Consequently, this also protects officers in the case of equipment failure: timestamp the activation / deactivation signals in a separate log file. If the footage isn't there, but they activated, then it's no longer their reputation on the line, because they had no involvement in the chain of custody.
[0] https://en.wikipedia.org/wiki/State_bureau_of_investigation
I agree it should be a black box. Rule #1 for accountability is that you don't rely on the entity that's being examined to give absolutely correct info. Whether that's debugging a process or reviewing an organization, outside instrumentation is essential.