If rim simply stuck to a single market then the company would stagnate and also the blackberry wouldn't exist as it wasn't their first product.
They are licensing their technology to the other phone makers, but while they are confident that they can make good hardware to fit a market then it makes no sense not to compete.
Incidentally, your analogy is rather wrong since the gap between the two devices will not be anywhere near as wide as that, realistically the 7290 and charm will be using fairly similar code with the obvious exception of the predictive text and other stuff that the charm will use.