Gotta love how Americans go against the grain by changing spellings, dropping letters, etc. Then they try to spin it like everyone else is odd for spelling it differently. We're on to you! ;-)
Right now on the front page there's a linguistics article stating that it was the British-English that diverged more, not the American version.
So, let's talk who 'changed' when you drop received pronunciation.
As far as the divergence in spelling is concerned that's almost entirely due to Noah Webster who purposefully spelled things in a more Germanic way because he found the English spellings with the French influences to be distasteful.(1)
[1] http://en.wikipedia.org/wiki/Noah_Webster#Blue_Backed_Spelle...
On the others, there's a bit more of a continuum than everyone using the British spellings, though I'll agree the Commonwealth probably tilts towards more similarity with the UK than with the US. But Canadian spelling, especially, aligns with U.S. usage in a lot of respects: it generally uses -ize rather than -ise endings ("realize", not "realise"), and sides with the Americans on a lot of specific one-off differences ("tire", not "tyre"), though it sides with the U.K. spellings on -our vs. -or ("colour", not "color").
Hey, I'm half American and half Australian so I don't really have a horse in this race (or perhaps I have two) but the provincial attitudes of some Americans when it comes to language and dialect differences is annoying.
I live in China and have spent most my adult life in a Mandarin/Hokkien language environment. Spellings and usage here generally follow the US, but very few people could even be considered to be fluent speakers. Hong Kong does still use UK spellings, but Taiwan and Singapore have considerably more US (and in the case of Taiwan, Canadian) influence. Japan and Korea also favor US spellings. So do Mexico, Guatemala, Nicaragua, Honduras and a lot of other places you may never have visited.
Maybe you see the attitude as "provincial" because you haven't seen much of the world yourself!
In any case, from a purely pragmatic view, the US usage is standard. I know the minority of native speakers on the other side dislike it, but let's face it. Just about everyone on earth is exposed to Hollywood and US television. Even in former UK colonies such as HK or India, North American usage is widely understood. The same can't be said for Irish/British/Welsh/Australian dialects.
It was the Americans, and Webster in particular, who decided to go around rationalizing the way things were spelled. It was the English who decided to go non-rhotic. We're both changing the language, and we continue changing the language every year!