Random Kung Fu Scientist

So I'm sitting here watching Alias (season 3, a very cool show) and I am struck by a thought. The show is about a female superspy and this episode starts with her kicking some serious butt, beating up at least five guys on her own, unarmed. Last night I watched Judge Dredd (a terrible movie, it was AWESOME), and at the end Stallone, the star, fought the bad guy while the two women paired off against each other. My thought is this - why do most action movies insist on women only fighting other women. In Judge Dredd, one woman was a highly trained cop, and the other woman was a scientist who just happened to know kung fu and could fight. I'm not saying it isn't possible, but I am saying that there was nothing about the character that suggested that she had these abilities until the very end of the movie. And yet, in Alias, it is normal to see Jennifer Garner beat the crap out of men and women, and it doesn't seem wrong to see the men trying to beat her up.
For some reason that has always bothered me, the way that women have to fight other women. If a woman is the star of the movie, then a woman has to be the villain. Why can't women be the hero and take out the guy. It might even be more cool to see a smaller woman take out a huge guy. Imagine if Cameron Diaz got into it with The Rock. You look at it and think The Rock should win cuz he's huge and Diaz sure isn't that large. But it would be totally possible to make Diaz win the fight in a completely believable way (most marial arts are based, I think, on using an opponents weight and size against him, so it's not necessarily the power that has to win, but speed, strength, and endurance), and it would make her triumph that much more impressive. But Hollywood doesn't seem to roll that way. And that's too bad.
Sidenote, The Rock is pretty awesome in Be Cool. I'll tell you about my little movie marathon later.