Here is the text from that link:
This is caused by a bug in the Amazon software. You can work around it by going to Preferences->Output Options->MOBI output and setting the “Enable sharing of book content” option. If you are reconverting a previously converted book, you will also have to enable the option in the conversion dialog for that individual book (as per book conversion settings are saved and take precedence).
Note that doing this will mean that the generated MOBI will show up under personal documents instead of Books on the Kindle Fire and Amazon whispersync will not work, but the covers will. It’s your choice which functionality is more important to you. I encourage you to contact Amazon and ask them to fix this bug.
Is this really a bug in Amazon software or just they way they chose to implement? I'm wondering what the bug is that we need to ask Amazon to fix. Is the bug that we can't sync personal documents that we side load? Is Amazon going to say that it is their design (that we happen to think sucks) or is there really something broken (e.g. that they would agree they need to fix)?