This is the way the CSS spec was written. Firefox (and then KHTML) specifically went against the spec. This is because they didn't have page zoom, and too many developers were using px because it was far more consistent than any other unit across browsers.

From CSS2 spec:

"Relative units are: * em: the 'font-size' of the relevant font * ex: the 'x-height' of the relevant font * px: pixels, relative to the viewing device"

Changing font size doesn't affect your device (browser), but zooming does.