Even if immigration laws are the MOST unjust laws today, that doesn't make them the equivalent of the Holocaust or slavery. Most unjust may not even be extremely unjust.
3. It is morally permissible to break an extremely unjust law. I can't disagree with this. It was morally permissible for those harboring slaves through the underground railroad; it was morally permissible for those who helped Jews during the Holocaust.
4. It is morally permissible to evade punishment for breaking an extremely unjust law. By agreeing to 3, I am also agreeing to 4. And I feel fine with it.
5. It is morally impermissible to enforce an extremely unjust law. This makes me uneasy. The moral permissibility of evading a law says nothing about the moral IMpermissibility of enforcing it. Also, moral permissibility does not OBLIGATE action by a person. Yes, it may be morally permissible to help slaves flee to the north; however, this does not require all people to do so. Statement 5 suddenly obligates action and I don't think the prior 4 statements give ground to make this leap.
6. It is morally permissible to punish a person for enforcing an extremely unjust law. Again, there are obligations for action here that cannot flow from earlier statements about moral permissibility. In order for there to be action, there must be a moral REQUIREMENT, and Bryan never takes his theses to this level.
I don't mean this to be a defense of immigration laws, but there is something incorrect about comparing them to slavery. They are probably the most unjust laws today; nonetheless, this does not make them necessarily as unjust as the most unjust laws in history. Justice seems to be an area where we are growing, slowly but surely, beyond the limits set by our ancestors. I don't think extreme statements help that progress continue, and thus I offer my critique of Caplan.