While sitting is correlated with mortality, I don't think it's been proved that it causes it. I suspect the relation between the two is more complicated than that.
As far as stress free... Interacting directly with many customers can be a trying experience. I think it can be a lot more stressful than an office job for many people.
I'm not saying you'll be dead in 5 years time if you spend more than 12 hours a day sitting on a chair programming. But it adds up if you're doing that your whole lifetime and neglect healthy food and physical activity. Of course it differs from person to person but I think you see my point.
Regarding stress free comment... I guess it depends on the person in question. Besides, when you finish for the day, you really finish for the day and that's the most important thing. Entrepreneurship is not for everyone.
Smart people will put health before entrepreneurship, especially if it prolongs their life and not just in the sense how many years will they have to life.
As far as stress free... Interacting directly with many customers can be a trying experience. I think it can be a lot more stressful than an office job for many people.