Assumption #1: The DPJ electoral victory had nothing to do with alliance policy: Although domestic Japanese issues dominated the August 30 elections, the Japanese public was also moved by appeals for overall "regime change." There was the sense that Japan was moving in the wrong direction. This belief was linked to Tokyo's post-9/11 alliance policy.
When the DPJ talked about its unwillingness to support any more "American Wars," it did so because it resonated with the general public. More than once I have had young Japanese ask me, "Why do Americans like war?"
In short, the public rejection of the LDP was in part a rejection of the Bush-era "Britain of the East" military-centered model for the US-Japan alliance.
Assumption #2: The alliance burden on Japan is low: The Japanese do not believe that they receive high benefits from the US alliance at a very low cost. If you consider both direct as well as opportunity costs, Japan actually pays quite a lot for the alliance.
For example, when the Bush administration pursued its questionable 2003 war against Iraq, they expected the Japanese government's full support even though 80% of the public were opposed. The DPJ is responding, finally, to Japanese public opinion in a way that the LDP-to its cost-would not.
Assumption #3: The DPJ doesn't support the US alliance: If the DPJ really didn't support the alliance, they are currently in a strong enough domestic political position to openly say so.
They DO support the alliance, but demand that it must change the way it functions. They expect the right to make up their own minds about what constitutes the Japanese national interest, rather than have it decided for them by Washington.
In short, anger at the DPJ is misplaced. The politicians only reflect the realities of Japanese public opinion, the people who democratically elected them.
Maybe the lesson here is that US administrations will now have to work harder to appeal to the Japanese people whereas in the past they had only to appease a small, self-selected group of conservative politicians and bureaucrats.