Why do people not understand it is a federal crime to come to this country illegally?

What they don't get, that really irks me, is why this law is now beng enforced. It is not about some flippant and sudden fear for safety. It is about actual violent crimes being committed by Mexican drug cartels.. violence that is spilling over the border into the United States. American citizens living near the AZ/Mex border have already been killed by these particular illegals. The reason AZ is wanting to now enforce this law is to curb that particular violence. It just baffles me how people can get all up in arms over an issue and yet be so freakin' uninformed about that issue.