Because a table isn't a graphical element. It's a textual element.

In this case, we're (well, I'm) not saying IE is being lenient. It's just rendering it differently. HTML was designed so that different renderings would both be correct. It was never intended for multiple browsers to show the exact same thing. Of course, anal retentive designers decided they wanted that, plus non-binary graphics capability, and instead of generating a new markup language, they decided to hack the hell out of the existing one.

IMHO, if you want something to match those criteria, go use SVG.