Why did this "backwards" power jack pass ERC in EAGLE?
The power jack seems to clearly have a "+" terminal labelled


It's not a "+" terminal, it's the symbol's 'origin' crosshairs. If you look closely you should see one on most symbols, though often it is partially obscured by other elements in the drawing.

Here are some examples (taken from Sparkfun's tutorial Using EAGLE: Schematic):-