In a post earlier it was asked why cabin crew don't secure the cabin at the gate and do safety briefs there, well airport gates are precious real estate and time needs to be used eficiently, therefore taxy is the perfect 'wasted' time to perform this role.
As a pilot I try to taxy as smoothly as is possible, but there are circumstances where this isn't always possible.
It's for every passengers safety. And if a passenger stands, particularly in turbulence, they are not only putting the safety of other passengers at risk, but importantly, potentially forcing a member of crew to leave their seat to sit them back down, and I would be very annoyed if any crew were injured as a result of a obstonate passengers.