1. The President feels that this is the best way to solve the problem for the country. It would be extremely expensive to deport all of these people, and even if e had the money, how are you going to find them...go house to house demanding to see people's papers?
2. The President has more to consider with the immigration problem than just what is on the surface. The elections in Mexico and their outcome, along with potential worse problems on the border if a Marxist is elected and actively encourages border jumping with malicious intent. Right now we have enough of a problem; imagine what would happen if we had undercover Mexican soldiers and revolutionaries coming across with the funding of their government.
3. Chavez is actively backing the Mexican Marxist candidate. Try to imagine Mexico as not a problem but an active enemy, in league with Castro and Chavez.
4. Mexico ha a large supply of natural gas. We would like to continue to be able to buy it.
5. The immigration laws are so screwed up that unless we get a comprehensive reform bill passed, all of the rhetoric in the world will not make the Border Patrol able to get a handle on the situation.
There. Lots of reasons for the President's position, and none of them tacky.
I think saving social security is another reason Bush is in favor of eventual citizenship for illegals. The Dumbocrats and RINOs wouldn't let him fix the problem outright, so I think this is plan B.