In a post 9-11 world anyone can cross those borders. Terror cells have been discovered in Montreal, Quebec, Ontario, Ottowa, and various other Canadian cities. Clearly they are not there for Canada, they have one sinister objective on their minds, attacking the United States.

In Mexico drug dealers pass through so much cocaine, heroin, and marijuana every year, in fact in Arizona alone every year about 300 pounds of heroin, 2,000 pounds of cocaine, and 22,000 pounds of marijuana is siezed on the Arizona border alone. That does not count what gets through!
Also, think of the amounts pouring into New Mexico, Texas, and California every year!
People on the border say that they feel like they live in a warzone, many keep gaurd dogs and an arsenal of firearms. The often build fences around their homes and ranches as illegal immigrants have been known to kill livestock for food or mirth in general.
Most of these illegal immigrants are good hard-working people, but some are not, and this is an action for national security and general preservation of this country.