Sitting too much can cause health problems (not because of the sitting itself but the lack of exercise that often accompanies sit-down jobs). It's been fashionable to say 'sitting is killing people' and suchlike in the popular media.
“Even if you are doing the recommended amount of moderate to vigorous exercise, you will still have a higher risk of mortality if you’re spending too many hours sitting,” says Dr. JoAnn Manson, one of the study’s authors, and chief of preventive medicine at Harvard-affiliated Brigham and Women’s Hospital.
What is "too much"? So what should we do? In school, we studied in class by sitting on the school chair. That habit continue in campus. Also, some worker are forced to do their job by sitting (e.g. taxi driver, pilot).
Btw, The article said that 8+ hours and it applies to woman.
Sitting for long hours is 'claimed' to be bad for your health. Not that I am disputing the claim, I actually agree and have gone back and forth between standing desk and sitting desk. I say 'claimed' because I think more research is still required. Standing all day can be quite fatiguing as well.