I'm not sure the linked studies prove a protective effect from soy... didn't look at them all, but the few I checked were epidemiological. So, were there fewer cancers in the soy groups because of the soy consumption or because of some other factor soy eaters have in common?
Susan J. Komen for the Cure has a list of risk factors for breast cancer... soy is listed as "under study"... the explanation they provide is that some studies show a protective effect and others show a negative one. Individual links were not provided as far as I could tell though - just a long list of references that I do not have time to read through!
I am a breast cancer survivor and do not eat soy. My oncologist gave me the green light invoking the "Asians eat soy and have less breast cancer..." line. However, there is some controversy about the quantities/type of soy actually consumed in Asia. Also, soy seems to have many other negatives and few (if any) positives that I see no reason to take the risk.
BTW, I got breast cancer at 38 years old. No significant family history, brca negative. I was vegetarian from about age 19 until around 29 and ate lots of soy during this period. According to what I've read a cancerous tumor that can be felt has most likely been growing for about 8-10 years...