The problem is that it's largely a question of jurisdiction.
As non citizens immigrants are not necessarily bound by our laws, and can't be prosecuted under our normal system. Local and regional enforcement often send them to immigration officials, and the only punishment some of them receive for various crimes is deportation.
Obama has issued an executive order requiring immigration to release any illegal alien who claims protection under the not yet passed DREAM Act. Therefore any crimes which an illegal alien might commit that would normally be referred to immigration may simply result in that person being released back into society without punishment.
So there exists this loophole that can be used to get out of jail free, so to speak, since the normal law enforcement system cannot handle them, and the immigration system is no longer allowed to handle them.
In theory the DREAM Act has limits that should prevent criminally charged illegal aliens from claiming exemption under this act, but who knows whether the executive order, which is the only thing currently in effect, is comprehensive enough to close the various loopholes people can get through.
The real problem, therefore, is at Obama made an executive order prior to the various agencies getting together and determining the proper jurisdiction for criminally charged illegal aliens. If he had simply waited for congress to pass the dream act, or not pass it, then the law would have taken effect under the normal processes and the agencies would have been able to act on it.
Since its not law, they can't easily act on it, all they can really do is say, "The executive branch has ordered us not to prosecute a certain subset of cases, so we won't. If something falls through the cracks we can't patch the cracks, we have no legal authority or jurisdiction to do so. There's no law that covers these new issues."
