I think we are starting to confuse two different issues here. The original debate is why do Chinese language school differentiate between American Natives vs British Natives vs. Aussie Natives, etc. The reason for this, as JJ and/or Janice said, is pragmatic. While we can agree that both American English, British English, Australian English are all English, the colloquialism, mannerisms, euphemisms, the cultural references, even the vocabulary set used by different countries are all quite different.
As to what Onobaron said, unless I misread something, I didn't get the impression that laotou was stating one was inferior over the other, just how he believes they are different. By the way, it is (using a unscientific survey) agreed among the Chinese, (including Kunming locals) that Kunming hua and it's derivative, mapu hua, is vastly inferior to pu tong hua. This is not due to an objective assessment, but rather through indoctrination in grade school that the only proper language is pu tong hua. A lot of my Chinese friends do not prefer that their children learn their local dialect simply because they don't want their children stigmatized due to their accent.
Another point is that most people like Kiwi, is comparing the differences between American and British English to Shanghai putonghua and Beijing putonghua. I don't feel this is accurate.
A more accurate comparison is the difference between a main lander Chinese and a Taiwanese Chinese. Both would argue that they are speaking proper Chinese. However, if let say you were to learn your Chinese on the main land, however fluently you speak and is accepted by locals, if you were to speak the same way in Taiwan, you would probably be gawked at awkwardly simple because of the perceived bias. Now, I'm not saying there is a bias between American English and British English, just merely making a point. If you want to use your Chinese in Taiwan, you'd be better off learning from a language school that teaches Taiwanese Chinese. And if you are coming to main land China, you'd be better off learning from main land Chinese.
But that's only an suggestion and not a hard rule. I think too many people are taking this issue too personally.
Finally, to close out, to all of you old enough to remember, "Can we all just get along?" (People outside the US probably are scratching their heads now. See my point?)