Why is everyone agaisnt the national DNA database?

I'm not particularly against it, but I understand why many are. A big issue is that based on DNA, insurance companies and even employers can know your predisposition to particular illnesses. They can (or at least will soon) be able to tell you whether you will get such diseases as cancer or heart disease by a certain age. With that knowledge, imagine a person applying for health insurance and being told "no" or at least being charged astronomical premium because the company KNOWS that by a certain age they are going to acquire a very expensive illness. Similarly, a person applying for a job and the employer saying "We can't hire you, because we know that in a few years you are going to get sick and we will have to help pick up the bill."