There is a large number of creatures that migrate from North to South on the American Contitent. To a large extent, humans are migratory creatures.
One of the driving dynamics of the current immigration debate is this natural impulse for North South migration. I suspect that this tendency is especially strong in Central America which is long and skinny as opposed to the US which is wide in the middle.
People on the North South migration do have a different opinion of their actions than a person crossing a sea.
Immigration attitudes that may have worked well for people coming through Ellis Island will not work the same for people traveling North and South on the same Continent.
Even worse, we have a silly problem with linguistics. While we tend to see immigrants as "people coming to America." People born on the American Continents see themselves as Americans.
The people who we are preventing from traveling to el norte have an ancestry that migrated through el norte in the distant past.
Of course, to say that Mexicans should be citizens of the US simply because they live on the same continent is a bit absurd. However, debates about the current illegal immigration crisis should recognize the difference between the European emigration and north south migration.
Perhaps the PC word for the people from Mexico in the US without proper documentation should be "undocumented American" rather than "undocumented immigrant."