The previous questions still apply. When a problem becomes large enough to overwhelm all resources to handle it then another approach is needed. It could be thousands of unpaid traffic tickets, back taxes, overcrowded jails or illegal aliens that have been streaming across the border almost unchecked for 50 years. Bush, for good or ill is actually proposing an approach to finally address it with the tools we have available. It probably won't work but that doesn't mean that there is some silver bullet that will.
The simple fact is that most of this country may SAY they are against rewarding illegals this way but after they say it they just go to the door and pay the illegal that just mowed their yard or bussed their table. It makes no difference what the Fed policy is if local governments will to cooperate in detaing illegals when they are arrested. You will fid NO local LEO organizations in most states that are allowed to "round 'em up" because the local and state governments know the economic impact if they do.