https://www.uspsoig.gov/reports/audit-reports/us-postal-insp...

"The Postal Inspection Service’s activities must have an identified connection to the mail, postal crimes, or the security of Postal Service facilities or personnel (postal nexus) prior to commencing. However, the keywords used for iCOP in the proactive searches did not include any terms with a postal nexus"

Turns out it's supposed to be used to protect the integrity of the mail system, though "postal crimes" can cast a super wide net in the USPS purview.

It also turns out according to the USPS Office of the Inspector General that they don't do that.

"We determined that certain proactive searches iCOP conducted using an open-source intelligence tool from February to April 2021 exceeded the Postal Inspection Service’s law enforcement authority. Furthermore, we could not corroborate whether other work analysts completed from October 2018 through June 2021 was legally authorized"