Putting foreign nations in the critical path
The "go it alone" philosophy is because we've been burned in the past. Then again, Russia may have a good similar argument about America being in the critical path of the ISS for them and delaying it's completion by years because of shuttle problems.
Regardless, the US is at a crossroads with the shuttle. One more foam incident and these babies will be put to rest. Commitments can be met at a later date with future launch vehicles without risking the deaths of astronauts.