That is weird. I just downloaded the apps and registered them on the devices and the email addys were assigned. Never messed with anything on the website. Are your iPhones running an old version of the app somehow? IIRC the Kindle iPhone app got email addresses with an update back in December. I wonder if deregistering the app on device and re-registering it would do anything.
EDIT: Just did a quick Google and it looks like it was back in December for sure...
http://lifehacker.com/5870147/the-ki...including-pdfs