becuase you don't take a quick straight down drop, there is usually a slope that takes some of the pressure/stress out of the G's.
Also, you're not taking a sudden downward drop to your feet/legs and then another surge up. After talking to a guy at work, who is a physics major, in order be able to stand up on the ToT what would have to happen is this:
1) it would be one straight drop down, no double or triple drops.The pressure put on your legs/ feet could fracture a bone. In order to have these drops the pick ups would have to slow down to relieve some of the pressure.
2) the drop would have to slow down considerably towards the end of the ride for like two stories..this would reduce the pressure. So there really wouldn't be a sudden stop.
3) would have to strap down your legs somehow, otherwise they could be pulled from right under you. Thats why some drop rides have that little shelf you sit on -it helps to stablize you. So you might as well just sit down anyway.
He said it would be possible, but it would take ALOT of research time and testing plus redesigning that may 'cheapen' the thrill of the ride.