One slight correction: the MOBI and KF8 formats are not "mostly proprietary." They're entirely proprietary and owned by Amazon. Their software creating ebooks in their formats. Why shouldn't it be up to them what you can commercially do with either?
Also... "near monopoly" is rather meaningless in terms of legality.
As I said, I'm disappointed that Amazon has chosen to take this particular stance, but that doesn't affect my ability to recognize their legal right to do so.
The OP's client wants to sell their book in the mobi format from their own website (there is no need for them to do so). There are ways to do so without violating Kindlegen's ToU. That the client would prefer NOT to use those other options does not translate into "being oppressed by Amazon."
Last edited by DiapDealer; 03-27-2013 at 06:47 AM.