While I was waiting at my gate (after security check) to fly to San Francisco from Europe, the gate agent asked me to go to a "Document Check" station manned by other United Airlines employees. At this station, the employee asked me to show not just my covid test and passport, but also my permanent residence documents. Most questionably, he also asked about my itinerary and my relationship with my travel companions.
I was pretty shocked that an airline employee is allowed to ask such questions, so I complained to a United Airlines supervisor at the gate. The supervisor told me that these are "TSA questions" and that it is common for United employees to ask these questions.
This is the first time I experience such questioning after many flights back into the US. Is it truly allowed and common?