you lump all opposing viewpoints into the most extreme viewpoint and only argue against that.
You know it when it's used against you: It's what the anti-gun folks do to us. Instead of arguing about what constitutes legal and lawful licensed gun ownership they claim we want it to be the wild west, with access to anything and everything from simple pistols to flame throwers.
The truth is if you're willing to listen there are opposing viewpoints that aren't as extreme as you list here. There are plenty of people who believe in closed boarders but are against the idea of "The Wall". There are plenty of people who are for immigration reform without going as far as to bestow faux citizenship on illegals.
But you aren't interested in arguing or compromising with them. You'd rather lump them in with the crazies and argue against that because it's easier. Safer. Less challenging to our own viewpoints.
There ARE shades of grey. You're right: You don't compromise with the extremists... but if you assume EVERYBODY who disagrees with you is an extremist you turn into one yourself.