>If they are operating on US soil and citizens then there's basically a lack of oversight.
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OR - A deliberate intent to ignore what are supposed to be "The Rules", and use said agency's infrastructure for nefarious purposes to expand personal gain with no regard for the agency's initial and designed purpose.
This second option is far more likely, imo. There would in fact be MORE oversight required to keep all of the dancing minions in line and quiet.
>an agency has to have a mandate, otherwise why not just have one agency to do everything?
See above.