If it's simple statistics then anyone with a family history of cancer or heart disease or even diabetes would be passed over for employment. They're health relates to their ability to do their job. So statistically speaking they'd be more likely to require medical care and time off from work. Does this sound about right?
I wouldn't have any problem with a business adopting such policies, but the problem is they would be totally ineffective. Since medical records are protected information all the applicant would have to do is lie on his application and there would be no way to prove otherwise. If you're overweight on the other hand it's readily apparent and that's largely true of smokers too. Very few smokers can go 10 hours a day without a smoke and those that can probably aren't much of a health risk. Dopers can ussually be rooted out with piss tests.