It's a false positive, so to speak, and it's not an error, strictly speaking, much like -moz, -webkit CSS aren't. And the one I appreciate and tolerate, like <xml> element, that I, once again, feel it should have been a natural solution for many, many problems. I almost love <xml> and feel so sad it's not standard, not even in these new specs.
I prefer to use less:
<![if !(lt IE 7)]>
<!--[if gte IE 7]><!-->
which, by the way, is a dirty trick. Maybe clever, but dirty. I don't like this "smartiness" in my code.
It's for the same reason I don't agree with *+ html, for example, and I prefer a clean CC. I'm not making my omelet mixing rotten eggs with good ones. My unclean code on a side, my clean code on the other side. But it's a personal choice.