They aren't thinking that far ahead. It's mindless "yay freedom!", which is my own starting point 99% of the time, but not when it erodes the societal foundations that make liberty possible. Which is what happens when you import a significant chunk of people that are low-skill, low-education, poor, like a strong central government and/or Islam, etc.
Hell, even importing a bunch of micks and wops around the turn of the 20th century was enough to pave the way for big-spending, corrupt political machines and unions that became the backbone of the modern left and put us on the path to today's dumpster fire of governance. And the gap between 1890s Italians and 1890s Americans was waaaaaay smaller than between 2015 Syrians and 2015 westerners.
But no, I'm sure it will all work out great because some millenials really really really want to be nice to brown people.