This has probably already been said somewhere else but i don't believe its a functional issue with WGT but a procedural one. I am under the impression that the strict rules of golf say that you must complete a full round with the same type and colour of ball (I don't even know if you can change the number on it) and so WGT are adhering to that as closely as possible.
If you run out of a certain type of ball during an official formal competition (such as on the PGA Tour) I believe you are deemed to be unable to complete the round and you post a N/R.
Not saying I agree with the ruling but I believe thats why you default to the unlimited ball when you run out of one type/colour rather than go to an other type/colour as you would do in most of the amateur comps we all play in real life.