Bush has want an open border with mexico since before his first election. The attack on 9-11 put a hold on that and now he has his first real chance of getting it.
Perhaps the sovereignty of the U.S. is not a priority for you and others but it is to some. Those that don't believe in our sovereignty have either a personal stake or are products of the public schools that teach that nationalism is a bad thing.
So he went about it by talking loudly about it and planning to come up with a comprehensive progrram which includes a physical wall, cameras, infrared, UAVs, and other tools to help enforcemnet.
Most people wouldn't even be talking about immigration if hedidn't come up with his proposals. And if he really wants the borders to stay open, all he has to do is keep his mouth shut. The majority of Americans wouldn't be the wiser and certainly many FReepers wouldn't be able to criticize him.