The U.S. is perfectly nasty to immigrants coming from the "wrong" countries, especially Mexico and Central America. However the situations which they are fleeing from are even worse.
Im under the impression that Mexicans don't emigrate to the USA much anymore, most Spanish speaking immigrants are from Central and South America. Mexico has a lot of troubles, but other Latin American countries have it far worse.
Yes, America is that great and the proof is in the (immigration) pudding. There's a surprising gap between stated preferences vs revealed preferences in this area.
No comment on Mexico.
Either that, or it's disproportionately more appealing to people who are actually inclined to migrate. There is actually some reason to believe this is true, though I uh, wouldn't necessarily bet on it.