I agree with C yu that the caps help distinguish the title bar text from the menu text and also help it look a bit cleaner because the equal height of the letters give it more visual contrast with the rest of the interface.
Taste aside, I'm not sure why people would get so irritated about this as a design choice. You know how many websites use uppercase for top-level menu titles? Just look at Channel 9.
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onoz Channel 9 is yelling at us