On noone from USPS having won a stage except for LA, here are some considerations that suggest that argument is flawed and/or otherwise does not emphasize the crucial points:
-- Unlike other teams, USPS has as its sole focus LA's winning the GC at the end of the Tour. It is energy-consuming for domestiques to try for stage wins/other honors, and that takes away from the team's focus and the saving of energy to assist LA. Every bit of energy counts, for it count be the difference between the domestique covering LA during a crucial period at the end of the stage or falling off before that.
-- It is best for LA to win stages in many circumstances because he is strongest USPS rider and he gets the time bonus of 20", thereby further building up his lead in the GC.
-- The very distinction between a team leader (LA) and his domestiques is that the team leader is the person that gets the most of the external awards (or at least tries to). Otherwise, why would the team leader be team leader?
Especially at USPS, everybody understands they should have no personal ambitions at the Tour and they have to ride for LA.
-- Even in situations where LA is considering gifting a stage to a domestique, there are often third parties involved who are close enough to "steal" the stage from the USPS. In that situation, such as Heras/Beloki in 2002, or Landis/Kloden/JU/Basso recently, LA cannot risk having one of his competitors win the stage. Bruyneel wouldn't let him do that certainly.
-- The model adopted by USPS on the mountains of each domestique's pull being a "rocket" to propel LA for a while (per LA's book) tends to cause the domestiques to expend a lot of energy, and therefore not be primed to contest for the stage win. Even if they had the energy, there is a question of whether they should save it for future stages to assist LA.