Adam Clarke on Exodus 22:21

Thou shalt neither vex a stranger, nor oppress him - This was not only a very humane law, but it was also the offspring of a sound policy: "Do not vex a stranger; remember ye were strangers. Do not oppress a stranger; remember ye were oppressed. Therefore do unto all men as ye would they should do to you." It was the produce of a sound policy: "Let strangers be well treated among you, and many will come to take refuge among you, and thus the strength of your country will be increased. If refugees of this kind be treated well, they will become proselytes to your religion, and thus their souls may be saved." In every point of view, therefore, justice, humanity, sound policy, and religion, say. Neither vex nor oppress a stranger.