Apple has always decided what it thinks is the best experience for its Mac users, even against conventional wisdom or established, known user preference. This has been frustrating for power users through the years, from lack of two button mice to the latest reductionist “flat design” trend in iOS7.
The latest “we know better” feature is the automatic document auto-saving (with no warning) in its home-grown applications, like Preview, TextEdit or the iWork apps. I don’t want to recall how many original photos have been destroyed after a session of quick exposure experimentation.
Good news: you can completely disable the auto-saving via this terminal command:
defaults write com.apple.Preview ApplePersistence -bool no
This comes from a helpful topic on StackExchange.