Something like that has nothing to do with nationality, though. By that exact same line of reasoning, if Themyscira or Atlantis was being attacked then Wonder Woman or Aquaman would want to save those countries first. All it suggests is personal connection, it has nothing to do at all with whether or not the group that they're in is more affiliated with one country or another. Taking that logic even further, what if New York and Chicago were attacked at the same time? None of the League members have family in New York, so they might went to take on Metropolis first...but that doesn't mean that they're all of a sudden the Justice League of New York. Individual members can do whatever they want when they're not on league duty but officially, attacks in America do not take priority for the Justice League. They were stationed on a satellite and then on the moon, not in any particular country, for a very specific reason. Not to mention, members of the League are called upon to decide between responsibility to their loved ones and their responsibility to the world all the time. Whenever Batman takes time out of his busy schedule to play detective on the moon, that's one more victim in Gotham city that might not receive his help. Every single moment that Aquaman plays hero with the landlubbers, he forsakes his duty to his own home, all for the greater good.