I have this same problem: even with the little hack that gives you a menu choice to choose "left" or "full" justification, every book -- new or old -- that I've sampled (downloaded a new sample from Amazon) in the last couple of weeks has had that option grayed out. It was NEVER grayed out before -- sometimes it didn't WORK, but you could always SELECT the "left" option. I find the full justification without dynamic hyphenation impossible to look at. I own three Kindle Keyboards, and they're all gathering dust now; I'm doing my reading on a Kobo Glo, which reliably lets me set my own justification preference for every book. Why typeface, leading, margins, and so forth are considered things the user should have control over, but justification ISN'T -- why that one attribute is something Amazon won't let us adjust to our own preference -- is beyond me.
Hugo Award-winning Science Fiction writer
Author of FlashForward