I don't personally care about yaoi being available on my Kindle because I'm not a fan, but I thought I'd give somebody in the customer service department a hard time about it anyway. I explained my concerns to the first rep, then got transferred and had to start over. But this is how the chat wrapped up (c/p'ed with names changed):

Me: Would Amazon Digital Services have pulled them if they violated some part of the content requirements or something?

AMAZON CUSTOMER SERVICE: Yes, there are numerous reason as to why it could have been pulled

AMAZON CUSTOMER SERVICE: I'm so sorry, but we can't offer any additional insight or action on this matter.

Me: Do you know anyone that could?

AMAZON CUSTOMER SERVICE: We are not given any other information on the matter. I'm sorry.

So emailing the Kindle department is probably the best bet in this case. I wonder if I'd get the same response if I tried again tomorrow.