Possibly because if the USG were caught reading foreign companies' documents on foreign soil it would be a violation of countless international treaties, which

Possibly because if the USG were caught reading foreign companies' documents on foreign soil it would be a violation of countless international treaties, which would make the ongoing trade and infosec tensions worse, versus "whoops, this landed on our doorstep, we can do whatever we want on our soil". The fact that this can be easily chocked up to a logistical error is good cover.