Lance's plan has been to stay ahead of Vinokourov, Ullrich and Basso, and let other folks do whatever they wish. That is probably a winning strategy, but one with risks. A guy like Kloden might have just one or two strong days in him, so letting him get ahead is not a problem. But, if a rider such as Kloden turns out to ride well for the rest of the Tour, letting him take a three or four minute lead is dangerous.
The problem is, Plan "B", chasing every rider, every day, is even a higher risk. Lance can not chase Kloden one day, and Voigt the next, and still have gas in the tank to chase Basso or Ullrich later in the week. So, Lance must make an assessment as to who will be in the "Top Three" on the morning of July 20th, stay ahead of those three fellas, and allow the "presumed" non-contenders ride on up the road.