As pointed out, WGT have adopted the "one ball rule" that covers all Championships and even some local golf club competitions. WGT use courses as per the set up from either the US Open or The Open and follow the rules thereof (albeit allow you to finish with a starters ball to allow you to complete a round).
We are given fair warning that it is our last ball, so if we ignore that, it is at our peril. Yes WGT could change the programming to use another ball, but as most players will buy a new sleeve, this is in their best interest. Then there is the cost of re-programming for no return (and I agree that this could be classed as "marketing" or goodwill).
One point I would like to make is all balls have variances, so by the time you make TM or higher, you are probably better off sticking to one ball to try and avheive consistency.
I am OK with the current system, but would like to see how many hits you have with a certain ball, BEFORE you tee off. Currently it is hit and miss (unless you have a better memory than me - quite probable lol). As long as we are aware of the rules, I don't think we can gripe too much about it - and yes I have been stung by it before as well - was my fault thinking I could get 18 holes out of an oily rag!