I agree completely. I suppose their thinking is that they won’t compromise on the means.
The problem is that among mainstream libertarians they don’t even see the problem with allowing free migration in a welfare state not to mention seeing any problem allowing migrants into a society of free association. Steve Sailer has pointed how many libertarian economists can’t apply even the most basic economics to immigration. http://www.vdare.com/articles/economists-on-immigration-whats-the-matter