First, because the status quo doesn't.
Our current policy of haphazard border patrol, intermittent raids and deportations, wrist-slapping fines against offending corporations, and total lack of worker documentation has brought us to our current state: A nation of 10 to 12 million illegal immigrants who help boost our economy in certain sectors while draining our nation's resources in many others. If nothing is done we'll see millions more cross our southern border looking for a better life but end up living in transitional limbo.
The answer? Amnesty.
Registering illegal immigrants is really the only workable model available to us. Turn these illegals into tax-paying residents. It's better for the hopeful families - they'll have an opportunity to gain real freedom and economic change. It's better for us - we'll experience economic growth and cultural stimulation. It's better than what we're doing now. Set a date. Sign 'em up. Give each person a tamper-proof identification card. And start them down the road to legal residency.
Mass deportation is simply impossible. Let's be realistic. It will never happen. There are too many people here already. But to stop the flood of undocumented new arrivals while this registration takes place, an unprecedented (but possible) tightening of the border must occur. Yes, build the fence. But don't half-butt it. Increase border patrol (and prosecute the rif raf who aid and abet illegals). And pressure Mexico to get her act together. (Okay, that one's a bit nebulous.)
Obviously we'll export back south the criminals, the uncooperative, and the lazy (no one comes to America and goes on the dole). Plus we'll have to negatively incent corporations who hire undocumented workers (by imposing "draconian fines" and prison time for the executives, in the words of Bill O'Reilly). Will this be easy? No. The easy thing is to do nothing and complain about amnesty. But look where that has got us.
Also see Worry Wart: Framing Immigration Law
Open Posted at: DragonLady's World; Committees of Correspondence; Right Wing Nation
Picture Source: Institute of Oral History