bneyla1- I would like to see a person be able to make ball substitutions in a game or practice that doesn't count for average or credits but I see the need for it in those games that count. It prevents a player from customizing ball selection from changing ball characteristics from hole to hole to take advantage. Spin here, added distance there, slower meter on the next hole.
As for the fact that you can switch to a different ball in real life, not if the rules committee for the course you are playing imposes a one-ball rule. That is what WGT has invoked here. When such a rule is applied, you are not allowed to choose your backup ball. You are just disqualified and your tournament is over.
The starter ball here is really not to be viewed as a backup ball. It is simply a provision by WGT to allow you to complete the round.