b0geybuster: This has been explained 1 million times.
Specifically, WGT (the Committee) uses the "One Ball Condition" set forth in Appendix I, Part C of the USGA Rules of Golf. Here, the rule has been modified to allow for the substitution of the Starter ball should a player run out of a specific brand and model. Otherwise, as b0gey points out, players would be disqualified.
Joey