Why aren't people upset with Apple for breaking a new app with an OS update? Unless Bria is using, say, private API calls, why is the burden on them to update their app every time Apple releases an OS update, and not on Apple to ensure backwards compatibility, especially when there is very little difference to the end user between the two versions of the OS?

You can still run Windows XP programs in the latest incarnation of Windows, 15 years later.