Why is because there are a lot of people using unicode to write special characters.
I believe there was an option in msgplus! 3 where you could choose if you wanted to save with or without unicode.
As that option is gone from the preferences window I believe patchou removed this option. So there is no way to disable unicode.
Maybe patchou can reenable this option in a later build.
EDIT: Cookie kinda stepped on my point there