> That's what parallel construction is for
Parallel construction relies on having other law enforcement agencies available with which to conduct the parallel evidence (re-)gathering.
How would that work in a foreign country? It seems at the very least we'd have to have FBI or similar in their country (which limits the possibilities to nations with agreements that permit this).
The U.S. LE agent abroad would then still need to be able to detain and search for the evidence to re-locate, all while maintaining U.S. rules of evidence, and all while not getting arrested by the host nation in the process. The latter sounds like a guarantee to me if you should even whisper a hint that "parallel construction" is taking place over there.
A counter-argument is that Constitutional protections are not all considered to apply on foreign soil by U.S. courts, but given that these actions would be taken by an agent of the USG I'm still not sure how such a case doesn't get thrown out immediately.
> And also note that options for action based on NSA surveillance aren't limited to arrest followed by criminal prosecution.
But nor are the options literally infinite.