You may be correct in what you say but it does not apply in this case. The reason that prices in the UK are as much as double those in the US for the same product is not just tax related.Vendors have been ducking and diving on this for years. They are all at it!
The poster above saw Gates evading the issue when questioned. Why didn't he just use your ready made excuse if M$ are not directly to blame? From a marketing perspective wouldn't it have been much easier for him to claim that it was our tax system to blame and therefore out of his control?
Jon Honeyball of PC Pro has been featuring instances of what amounts to price rigging by US software companies in the UK and Europe for a year or two and that's what it is ... price rigging.
We are getting ripped off plain and simple and in this case it is not by the tax man.