I agree that it's a sensible behavior -- I've always wondered why the 'px' unit even exists if it scales just like everything else. That said, what does the CSS standard say?
[1]Because 'px' doesn't refer to pixels.
[2]It refers to a unit that scales according to display resolution and usual distance from eye; essentially it's a measure of the angle subtended on your retina. This is useful because (in theory) 1px will look "the same size" (and be about as legible) on a mobile phone, on a laptop, on a desktop, or on a projector, whereas 1in or 1pt(=1/72in) will appear much smaller if further away. What 1px is not, however, is a "pixel".