I'm not American, but there is clearly a difference between people who have visas that have expired, and then people who cross the border illegally. People who overstay their visa is documented, as they have gone throug a legal process and been accepted by the US agencies to enter the country. Their faulth lies in staying for too long, but they were documented and allowed to enter the country in the first place. This means that you know exactly who they are, which also means you know they aren't dangerous, as they were accepted in the first place.

People who illegally cross the borders are completely unknown, which is why they're often referred to as "illegal aliens". You have no idea who they are, where they are and what they have done. They haven't been allowed into the country and they haven't got any documentation at all. Furthermore, there are a lot of associated crimes, such as drug/human/sex trafficking, which is very unique for the southern border of the US.

So comparing these two is like comparing apples to oranges.