I believe that a lot of the libertarian objection to immigration law comes from issues surrounding respect for private property. The argument, as I have seen it, is that if you own a piece of property, you also should be able to control who can and who can not be present on your piece of property.
But immigration law subverts this, and declares that certain people (namely, those who lack permission by the state) may not be present on your property, even if you invite them. This is a violation of property rights.
Now, in an anarcho-capitalist society, this would easily be handled by contract. If a bunch of property owners wanted to get together and form some sort of mutual defense collective, they would be free to do so. One of the conditions for joining the collective might be “by joining, you agree not to allow any ‘undocumented individuals’ access to your land.” However, in the current statist model, nobody ever agreed to any of this. It is a pre-existing arrangement forced upon us by the state. If the state is allowed to tell you who can and who can not be present on your land, do you really OWN the land?