Searay: What would be wrong if WGT did disqualify players who ran out of their chosen ball during a tournament and refused to purchase another sleeve? Players are never forced to complete any round using the free white rock. It is the players choice to do so.
If, for whatever reason, someone has only one or two purchased balls that they can play with, and they don't wish to purchase more of those balls; they should start the round with the white rock. Or at least start with a ball that they intend purchase again so they are never "forced" to buy something they don't want.
I have a large inventory of free balls with varying attributes that I don't use other than CTTH tournaments or fun play rounds. When they are at the end of their lives (red zone) I know that I should throw them out or potentially finish a round with a white rock. My choice. But with tournaments I play my favorite ball and expect to purchase more when my supply runs out. I also know enough to keep several thousand in credits available for those times when I find the water two or three times in the same round.