As Piper_ says, the majority of Kindle ebooks have DRM and in particular are encrypted to lock each file to a particular Kindle device or a single instance of a Kindle app.
If you want to use Calibre's ebook viewer (K4PC or whatever) for Kindle ebooks from a K2 you will need to strip the DRM. This is very likely legal in the US for personal use only. There are 3rd party plugins to Calibre that will strip the DRM from ebooks as they are imported (via Add Books). See Apprentice Alf's Blog for more info on DRM.
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