Why did flight NH175 divert back to Los Angeles?

In the news recently is All Nippon Airways flight NH175, which returned to LAX after four hours towards NRT when it was discovered that a passenger aboard did not have the correct ticket, although his ticket was nevertheless to Tokyo. Two points immediately come to mind:

• If a security issue was suspected, why did they not immediately begin diverting to the closest available airport that could accommodate a 777?
• If there was no security issue, why did they not simply continue to Tokyo?

Perhaps another way to state the overall question is to ask why ANA protocol necessitates a return to the originating airport in this case.

• That's actually very kind because if I boarded the wrong flight, I'd like to be let off where I boarded, rather than somewhere unfamiliar. This is especially important for a international flight since the passenger very likely does not have the traveling documents for the destination (Tokyo), so Tokyo is not a realistic option, unless the passenger OKs that. – user3528438 Dec 27 '17 at 17:43
• @user3528438 - it may be kind to the passenger that made the mistake, it punishes the other 300 passengers on the plane that didn't make a mistake and they were forced to fly 8 hours to nowhere, then had to make arrangements for missed connections, hotel stays, etc. I don't know what the fine in Japan is for an undocumented traveler, but I think it's under $5000 in the USA, or around$15 per passenger, and I bet it cost the airline much more than that to accommodate everyone. – Johnny Dec 27 '17 at 21:54
• News reports are stating definitively that there was a security consideration. As for diverting to the nearest airport, it's not impossible that LA was extremely close to being the nearest airport. – DJClayworth Dec 27 '17 at 22:57
• I doubt this question can be answered, as it’s a matter of company policy/decision making. I’m sure there was some communication back and forth between the pilots and the company after the mistake had been discovered. There’s no way of telling who made the call to turn back and why. – TomMcW Dec 28 '17 at 3:59
• @TomMcW you may be right. Or perhaps surprisingly there will be someone one day who can answer, if the question is still open. – Koyovis Dec 28 '17 at 21:22

You're already aware that the flight crew followed ANA's security protocol.

Yes the flight was closer to PANC, for example. Approximate location based on average ground speed and time it took to return to KLAX is shown below (to make sure there weren't any map projection antics).

(gcmap.com)

Security protocols are not public, but we can deduce from the timeline of the events what had happened.

The cabin crew became aware first (not the airline ops), which means the passenger was likely to be the one to bring the issue to them. Boarding a different painted airplane nowadays than the one on the ticket is no biggie with code-sharing and whatnot, honest mistake.

"While" on the way back, the airline ops discovered it was a mistake, when exactly, no reporter says. Maybe it was soon after.

Let's flip the question.

Why not KLAX?

• Does this passenger have their [perhaps suspicious] checked-in luggage on board? Probably not.
• Can we search them and their carry-ons? Yes.
• Did they come forward? Possibly.
• Do we have handcuffs on board? Yes.
• Can we overpower this one person? Yes.
• Are we willing to arrange (logistics) for their accommodation and ticket back to KLAX? No.
• Do we have different flight and cabin crews to takeover the plane in PANC if needed (duty hours)? Hard to arrange.
• How about arranging for the whole plane accommodation until said crew arrives? Harder to arrange.

Then proceed with protocol. Reacting and overreacting are different. In conclusion, there was no rush to quickly get rid of the passenger.

Addendum: Here by coincidence Alaska is also part of the US, a different route means you're going to burden another country with unneeded trouble, and as the police confirmed, there was no security risk (while the plane was headed back).