Hi Deana, I think there are two issues here.
One is that a lot of people do not understand statistics, which is what surgeons etc look at when they advise whether it is wiser to have a double mastectomy or a single one if you have cancer in one breast. The fact that both breasts have probably been subjected to the same factors that cause cancer in one breast does increases the risk (as you say) of later finding a second primary cancer, in the other breast. Having a double mastectomy is a riskier process than having a single mastectomy, and the risks need to be weighed against each other. There is a significant risk of serious heart problems that increases, as does the risk of lymphodema, pain problems, damage to the lung with removal of central nodes, etc. If there is only a slight risk of getting cancer in the other breast, and a higher risk of serious complications if you have a double mastectomy, it is often wiser to live with the slight risk.
The other thing is that an awful lot of people with breast cancer do not seem to understand that breast cancer metastasizes by stem-cell-like cancer cells, (which are able to switch between an innocuous form and a cancerous form), which can break off from the cancer while it is in the body or being removed from it, which then move around the body through the lymph and the blood. Each of these cells may eventually find a good place to settle and grow again into a new secondary tumor in the liver or lungs or brain or bone or other places. These women believe that so long as they have removed all the breast tissue where more primary breast cancers could grow, they will be safe from having any more breast cancer. Unfortunately they are wrong, and that is not the way it works. Having a double mastectomy is not a guarantee of no further breast cancer. especially if radiotherapy and/or chemo are recommended and refused.
We are so lucky that possible treatments have become so effective, so hormone therapy, herceptin-targeted therapy, and chemotherapy are all getting less dangerous and more effective, giving us the chance of a cure if these spreading cells are effectively mopped up, and to survive a good long time in many cases even if our cancers do manage to metastasize.
best wishes