Players have options:
1. Make sure you have enough 'I paid credits for' balls to finish the round.
2. Start the round with WGT's free ball.
In addition, during a game WGT posts a warning letting you know you just started using the last paid credits for ball of the type you are playing, and gives you an opportunity to get more of the same ball.
I am continually amazed that so many people expect WGT to take responsibility for the players actions, or in the case of running out of balls - a players lack of action.