It's most likely not Amazon calling the shots. You'd have to take it up with the various authors and publishers. Different countries have different laws, and sometimes regional publishers will decide their own release schedule based on the local market. It's like how a game can be released for the Nintendo 3DS in Japan but it won't be released in the US until a moth later. That's because Nintendo of America decides the release date for US products. Delays can also be caused by localization.
If this were the case, different countries, different laws etc. How did Kobo manage to overcome the same issues?