Why did ANA175 return to LAX rather than continuing to NRT or diverting to another airport? [closed]

Recently, a flight from Los Angeles to Tokyo turned around and returned to LAX after it was discovered that an unauthorized passenger was on board. From the LA Times:

A Tokyo-bound flight from Los Angeles that turned back to widespread attention on social media was carrying two brothers, one of whom was not authorized to be on board, the Transportation Security Administration said Wednesday.

The unauthorized passenger had a boarding pass for a United Airlines flight, but boarded the All Nippon Airlines flight with his brother, who had the correct ticket for the flight, said Mike England, a TSA spokesman.

...

“At the time during the flight, the pilot in command was presented with information about the discrepancy in the passenger manifest. Based on the available information in flight, he made the correct decision to return to LAX,” the airline said in a statement. “ANA supports the decision of the pilot, out of the abundance of caution and safety for the passengers and crew onboard.”

From my armchair, it seems like a drastic step to return all the way to LAX rather than diverting to an airport such as Anchorage or Seattle, or even continuing to Tokyo. I'm aware that under some circumstances, airlines can be fined for bringing passengers to a country who turn out to be inadmissible; but I can't imagine that this fine is greater than the cost of scotching an entire intercontinental flight.* Similarly, it doesn't seem likely to me that the additional expense of a diversion would be less than the cost of returning to LAX.

Turning the flight around for security reasons doesn't entirely make sense to me either. The flight had already been en route for four hours before the error was discovered; the additional risk taken on by continuing or diverting seems minimal given the level of risk the flight was already exposed to by letting the extra passenger on in the first place.

But I'm not an expert in the cost-benefit-risk analyses that go into these decisions. So I ask the experts here: What factors would be considered in making a decision like this, and how did these factors lead to this decision?

*In the end, the extra passenger would probably have been admissible—he had a boarding pass for a United airlines flight from LAX–NRT leaving at the same time—but the crew presumably did not know about this at the time.