If you make a change to an email preference, and submit the form, the change goes away. Everyone gets defaults, whether they like them or not.
I expect that someday we are going to upgrade to 2.17.x or 2.18.x bugzilla, and that this problem will spontaneously go away. I don't really have a good insight about why it is there, and it's not a big enough problem for me to worry about anymore, now that we have llvmbugs@cs.uiuc.edu working satisfactorily.