Igorsk's tool is not illegal - it's not a DRM circumvention tool. It allows you to buy any DRM-protected book in a Mobi store, using the perfectly legitimate Mobi PID that your Kindle has, and then makes a very minor "tweak" to the book's header to allow the Kindle to read it. Absolutely not illegal.
The tools you are referring to such a "MobiDeDRM" are absolutely illegal in most countries, and personally I wouldn't touch such tools with the proverbial barge-pole.
Again, there are not "two formats". Kindle books ARE Mobipocket books - full stop!
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