If you've got untreatable cancer then yeah you should be forced to disclose. However cancer invokes more fear than there is actual risk, if I get skin cancer its 100% treatable, some forms don't even need treating because its merely a skin growth with no health risk except maybe an infection if you cut it off with a blunt knife. Yet if a CEO announced they had 'skin cancer' the stock would drop, even if it is one of the benign forms that doctors treat with wart removers.

I think saving the investors from themselves should be just as big a concern. He is predicted with survival past 10 years, which IIRC the average age of death with someone having that prognosis is in fact only a few years shorter than the average age of death. If Jobs getting cancer means he kicks it at 78 instead of 82, I don't see why it should be disclosed immediately or at all.