[tpz]

Not quite. Japan can and does defend itself just fine against neighbours like North Korea, Russia, and China.

The American taxpayer does something different than you state. In actuality, they fund the projection of force everywhere that the US has interests. To relate back to the example of Japan, this means that the American taxpayer funds the defense of the things the US wants from Japan, not truly the defense of Japan itself though the defense of the former can often aid the defense of the latter. This holds true essentially everywhere else the US chooses to project force and the distinction is absolutely critical but too easily forgotten.

EDIT: in case the above sounds a bit too much like some kind of anti-American rant, the point I should have made more explicit in response to the parent is this: parent makes it sound like Japan is restricted from defending itself against the listed neighbour countries and therefore requires outsiders to do so. This is not true, and since it is not true, one should periodically ask themselves why the US has forces there (and everywhere else.)

[philwelch]

There's a much more boring reason behind it too, though: Japan's military isn't nearly strong enough to hold off North Korea or China by itself, and if Japan's military was strong enough to match its neighbors, all the other countries in Asia would be very uncomfortable about it. It just keeps everyone happy (and trade flowing) to keep Japan militarily weak, and the best way to do that is to guarantee them the protection of our military.