Christ. The haka can lead to fights, thats just whats up (Ask Tipoki). People can respond in any way that they want but it was decided years ago that the best way to prevent dust-ups during these heated moments was to keep both sides in their respective corners and out of punching radius. Both teams behind their 10 metre line, thats the best way to handle it.
If the opposition want to respond by staring angrily or with their own haka, with a song, with a breakdance routine, anything, they can do that. The only rule is that they do whatever they want to do behind the line. They can even walk off or not watch if they want.
Its hard to explain the atmosphere that arises when a haka battle gets too close, its sort of like that uncomfortably tense moment when you can see a fight on the cards and all it would take is for a slight twitch and all hell will start breaking loose.
This is why the rule is there. One wrong move away from carnage, and thats only schoolboys.