Do you know that the Japanese Self-Defense Forces are already the 4th biggest budget of any army in the
world after the US, China and France? Moreover, Japan is far from being unique in having US troops stationed on its land. As I have explained in
this thread, there are over 230,000 US troops stationed in 130 countries around the world. That means that few countries (probably the "enemies" of the USA) do not have US troops on their land. There are 117,000 US soldiers stationed in Europe, including 75,600 in Germany, compared to only 45,000 US soldiers in Japan and 37,500 in South Korea. Considering the USA's tremendous technological advantage, it would be foolish for a country that can benefit from their protection not to take it. Moreover, US soldiers buy many goods and services around where they are stationed. Even the most anti-US Okinawans think twice about asking US troops to leave, as it would seriously blow the local economy.
I personally don't care whether US troops stay in Japan or not. It does not directly affect me. I also don't understand why people like you make such a big deal of it. Why is it that the Japanese always find all kinds of crimes or misdeeds committed by US forces, while we never hear similar stories in Europe? Germany (pop. 80m => 1 US soldier per 1,050 pers.) has proportionally 2.8x more US troops per inhabitant on its soil than Japan (pop. 128m => 1 US soldier per 2,850 pers.), but nobody seems to complain.
If you haven't done it yet, you may be interested in reading the thread
Why are U.S forces still in Japan ?