Since OS X Mountain Lion, the Mac defaults to preventing applications from unidentified developers or sources from being launched. You’ll discover the message in OS X 10.8 when you try to launch BookingCenter for the first time. You’ll get an alert dialog that says “BookingCenter can’t be opened because it is from an unidentified developer”.

Why don't we just become an Apple 'identified developer'?  Because Apple sant hundreds of dollars per year to include the app in their 'app store' and the Desktop model is dying, so the cost isn't worth the reward.  Not to mention, we all pay Apple a lot of money each year as it is :-)

This new security feature is called GateKeeper, and it doesn’t mean you can’t run BookingCenter (or any unverified app) on the Mac, you just have to either temporarily skirt the security blanket of GateKeeper, or turn off the app limitations entirely.  There are instructions for doing so at: http://osxdaily.com/2012/07/27/app-cant-be-opened-because-it-is-from-an-unidentified-developer/

Note, this only affects Mountain Lion and later versions of OS X.