Interestingly enough, there was a recent randomised, controlled study of green tea extract and prostate cancer that seems to have been highly successful, although the numbers of participants was small.
See "Green Tea Prevents Prostate Cancer" at American Association of Cancer Research website:
http://www.aacr.org/Default.aspx?p=1066&d=432
Also, the epidemiological evidence for green tea and breast cancer generally indicates that it works against recurrence, not the initiation of cancer, although there are some problems (the benefit does not seem to go up with increased consumption).
I guess that the FDA doesn't want merchants hawking green tea based on weak scientific evidence, but I personally figure that if it even might have an effect it is worthwhile as a chemopreventative (preventing more chemo!).
The U.S. National Cancer Institute is carrying out tests on these things in large properly conducted trials that should settle this once in for all, but I suppose that nothing will be approvable until then.