Amazing . Your account is over 7 years old and you have only as many posts . So in the time of your absence this question has been asked over 100 times . But since you dont visit the forums regularly here goes .
The exttra balls you have in your inventory arent in your bag after you start a round . When the game gives you the option to check your clubs that is the time to do so . Whatever ball brand that you start the game with that is what you will have . Once you run out the game gives you the choice to buy more of the same brand or finish with the rock . The game does not go back into your inventory and give you another ball of your choice .
Consider those balls as being left back in the locker room or the trunk of your car . Since your account was started in 2009 you should know the answer to your post by now . So for your own good never start a round with 1-2 balls in your bag . You have to allow for ball loss due to water shots or going out of bounds .
My suggestion to WGT was to kill the round when players run out of balls and that would end the problem . Give the player the option of either buying the brand they were playing with or player forfeits the round , instead of finishing with the free ball .
So there you go Roy the answer to your question as to the why not . Sorry that it doesnt help and that it also wont change a thing .