What happen if you have a free ball that you don't want to buy, or if you don't have enough credits to buy the ball you were using?
Actually, the rules of golf allow you to switch balls during the round, even on the same hole, and it doesn't have to be even the same type of ball, just the same brand, except in tournament play when the "one ball" rule is in effect. In the case of WGT rounds, I have not seen the "one ball" rule stated on any tournaments.
So if I am playing my Callaway ball and have no more balls in my bag of that type but have a sleeve of similar balls, and I have no more credits and don't wish to spend any real cash on new balls, WGT replaces my Callaway with a starter ball. This is, by definition, a violation of the one ball rule - I am replacing my ball with a different brand and type. WGT may wish to give the impression it is following the rules of real golf, but it is only stacking the deck so that people will spend more money on them. Why complain, it's not real life, so if your score suffers because you are too cheap (I happen to be one of those types), then just suck it up and take your lumps.