Amazon complained about KindlePID, which is needed to read OverDrive MOBI ebooks without removing DRM. They did not complain about mobidedrm, which allows you to strip the DRM from any MOBI ebook. In this case, you already know the PID because you gave it to your ebook store (e.g. use the PID from Windows MobiPocket Reader). Stripping DRM from ebooks is generally thought to be illegal in the US, although this has never been tested in court and doing so for personal use on ebooks you bought harms no one.
In fact, if you don't mind stripping DRM, you can buy any popular ebook format because they have all been circumvented. MOBI is the easiest, since a DRM-free MOBI is directly readable on your Kindle. However, many UK ebooks (for example) are only available as Adobe ePub (see
Adobe ADEPT DRM for EPUB circumvented). Once an ebook is DRM-free, Calibre can format shift it to MOBI.