Ah, but PNG is an image format that is widely used by open source enthusiasts, in reaction to restrictive patent rules surrounding GIF.
As a historical note:
Way back when, MS decided that it would make its browser pretend to web servers that it was Netscape - why is that? Because it stopped servers maintaining separate versions of the site for IE and MS, that took into account IE's many bugs. Then IE users just saw a broken site, complained, and forced server admins to make every version of the site take IE bugs into account - ignoring the (at the time) superior capabilities of Netscape. Since nobody got to see the (presumably) better designed NS versions of webpages, the superiority of NS wasn't recognized very widely. Thus is a browser war won...
So, why cripple PNG - because those evil Linux types use it. People are hardly going to move over to Linux if they think, "those stupid Linux people, they can't even make a website that works"
More likely, MS doesn't care enough about OS X to fix all their bugs.