Each company is different, and from what I can tell is that you were an outside sales consultant for a company similar to Avon or Mary Kay.
I am sure that there is something in the sales agreement that you might have signed years ago, or something they have written that states if you don't order the product for over a certain amount of time, then you start fromt he beginning, etc.
I don't think that they are doing this to you becuse you had cancer, but, because of the lack of ordering of product.
You might want to appeal to the company, maybe if they hear the reason you were not able to sell their product, they might make an exception.
It's not legal to terminate someone due to health problems. However, companies have a way of rewording things to make it seem it's not health related. They could say they had to downsize or they terminated that particular position. Then they hire someone else and give them a slightly different title. One person I know was let go because of "restructuring." But they hired another person shortly after who had almost the same identical duties, just a different name.
If you are so ill that you cannot do the duties for which you were hired, they may have you there. I think they are supposed to make some adjustments if possible to accommodate you. This gets into legal issues. There should be a legal consultant at the firm that can advise you. I think most larger businesses do have some kind of employee advocate in place.