Is it? That may be true, but I read about new (to me) contradictory research in this area a lot. I feel the science should be more settled before we start designing specific policy proposals.
Research that contradicts benefits of frequent physical activity for office workers? Paint me surprised. Can you share some links?
I'm pretty sure I remember reading that the effects of sitting were not, they believe, offset by physical activity. I can't remember the source, but I'm sure there's stuff out there in Google.
Pretty sure I remember those same studies. More or less, I believe it was that the effects of sitting all day were not offset by physical activity after work or on weekends. The conclusions were not that taking breaks from sitting have no positive effect.
There is certainly research that standing for longish periods of time is bad, so neither sitting nor standing desks are really a solution, but I would be very surprised if moving were bad.