Scott - IRL the tournament committee has the right to impose what's called the "one ball" rule limiting the player to exactly the same ball brand, model and color for the duration of the round. WGT chose to implement the same in this game. My guess is that it's to (1) prevent players from changing balls each hole to one that is favorable on that particular hole, and (2) increase ball sales.
IRL the player is disqualified if unable to continue with the same ball. Instead of the loss of green fees, tourney entry fees, etc. WGT gives us the option of buying more of those balls before the last one runs out (or is lost) OR finishing the round with the free ball. The free ball is a compromise with the IRL rule rather than just throw you out of the round.
I don't mind the rule in ranked rounds because I always make sure my golf bag is stocked with enough equipment prior to a round but I do wish they would not enforce it in practice rounds. As IRL, it would be nice to use the mulligan feature to compare different balls in different situations. No "one ball" rule in practice would greatly help people learn the game play and the equipment.