Why can't China do something about their crazy aunt in the attic? hint: they're bankrolling these launches.
To see if we react and how. It's like getting your little cousin to throw a rock at your neighbors house and letting him get in trouble for it. If no consequences, it's only a matter of time and you'll be able to do the same with impunity.
I heard a fellow on the Glenn Beck show yesterday call in and say that every time North Korea launches something, the US radar systems (and presumably the defense systems in Alaska) light up to track the NK missile.
He said the Chinese watch this with their own systems (including satellites) to determine ways to evade or neutralize them. I don't know how accurate this comment is, but Beck said he would investigate it.
If this is true, then China and NK are in cahoots, as most of us think they are.
China has another interest here - to let the USA spend a portion of our treasure on defense systems and on bases in South Korea. They figure, I think, that they can always get someone to give them the design and operational secrets when the defense systems are developed - just like they did with missile guidance systems in the Clinton years.