In short, it's because of the IE8 parsing engine.
Guess why Microsoft has trouble working with the new HTML5 tags (like "section") too? It's because MS decided that they will not use regular XML parsing, like the rest of the world. Yes, that's right - they did a ton of propaganda on XML but in the end, they fall back on a "stupid" parsing engine looking for "known tags" and messing stuff up when something new comes around.
Same goes for IE8 and the jquery issue with "load", "get" and "post". Again, Microsoft decided to "walk their own path" with version 8. Hoping that they solve(d) this in IE9, the only current option is to fall back on IE7 parsing with <meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
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Oh well... what a surprise that Microsoft made us post stuff on forums again. ;)