A liberal state must allow freedom of movement
Those who are not currently citizens of a particular liberal state should enjoy the same rights are those who are already citizens. When a liberal state disallows non-citizens entry into its territory, it negates their liberal freedom of movement and behaves inconsistently.
It is a mark of the liberal state that it grants certain rights to its citizens, including the right to go wherever they please. States award this right of freedom of movement to its citizens by virtue of their humanity. Every human being should enjoy certain freedoms, such as the freedom to worship whichever god they desire or the freedom to go wherever they want. The state should respect and protect these rights. This is a central tenet of political liberalism. Those who are not currently citizens of a particular liberal state are still humans. Thus they should enjoy the same rights are those who are already citizens. It would be inconsistent for the liberal state to deny this. If a liberal state disallows a non-citizen entry into its territory, then it will have denied his or her right to freedom of movement and fallen into inconsistency.
Rejecting the premises
Finish this argument and its counter(s) using this source: https://plato.stanford.edu/entries/immigration/#Lib Another source: http://www.jesp.org/PDF/Liberalism%20or%20Immigration%20Restrictions_final.pdf