This is to do with the old "Select" program when you had to specifically opt out or it is automatically renewed, like it or not.
I used them ONCE in the early days and lost money during that 90 day period. I went from 100+ monthly income on a title to 3.00 for three months.
Also, the website was sneaky, automatically enrolling a book into the exclusivity deal. You had to check a box to
not be enrolled! I only twigged to that the third time I read things through.
They did not allow a "do over" then, either. Once in, you were stuck with them for the duration.
Things may have changed by now, but the damage is done, I do not trust them.
In ANY contract, the wording is not going to be slanted in favor of the writer, so read carefully and make sure you thoroughly understand the terms. If not, then AW is here to clarify.
Some are okay to deal just with Kindle, but I prefer having my works in multiple digital venues like Smashwords, B&N, Kobo, etc. Your book can be in ONE store or several. It just makes more sense to choose the latter.