My Really Dumb Question About That "Wall"

I may have gotten this all mixed up, but I have a question to those who don't want the American Government to construct a wall to keep out illegal immigrants.

Maybe I'm just stupidly simplistic, but if I understand those against the "wall" correctly, they think that anyone who wants to move/migrate to the United States should be able to, whenever they want, without being forced to go through the legal, bureaucratic procedures. If I understand correctly, those against the "wall" consider what they call "migrating" as a human right.

So if "migrating" shouldn't be a difficult hassle, then why should there be border control anyplace? Why should tourists, who just come to spend money, enrich the American economy have to worry about visas, passports and entrance laws? And what if some of those tourists decide to stay in America, shouldn't they be welcomed with open arms and allowed to live in the USA without fear of being deported? What makes the hispanics coming from the south of the border be more desirable as "new Americans," than those who fly in, or enter any other way?

Just curious. Did I misunderstand something? Or am I just stupid?

New York City