As a business, they try to please as many people as possible. This is shown on updates and maintanence the most.
There main player base is asia (asians statistically enjoy this type of game more then anybody else), we then have the US market second. After this, you could put Europe, Canada, Austrailia, etc.
Therefore since about 60-70% of there Client base (just a number, but could be higher) is in asia/US, these are the customers they are going to attempt to please the most.
There have been maintenance directly in japan prime time, and there has definatly been downtime directly in US time, it just happens proportionatly less, since this is their main client base.
The reason you think it is always on your time zone, is because it affects you more when it is in your primetime. From what I have said, I will agree it is in your prime more, but this is because of statics that they have gathered over the years, and the least amount of logins ACCROSS THE WORLD tends to happen after Japan midnight (11AM est), therefore this is where they are going to put normal maintanance).
I am canadian, and they have had their last 3 major updates directly on a holiday for us (thanksgiving (a few days ago), august long weekend, July 1st (canada day), but it isnt a holiday for the majority, so it makes sence. The update on monday was a holiday in japan as well.
Believe me if they didnt care about us, would they really be giving us updates according to what people want/demand (ie. npc's, ranger updates, ninja updates, new endgame material, new rare/ex drops of monopilized NM items etc.)
There will always be people upset about downtime, but for the about 15 cents that this 10 hours is going to make up of your 12 bucks a month, I really dont think it is all too bad. People need to calm down a little, since every single maintenance there is a whole list of people here that get upset over something trivial like not being able to play a GAME for a few hours.