The Aruch L’ner(Teshuvos Binyan Tzion 2:60) challenges this explanation. He cites the Gemara (Sanhedrin 102b) which states that Imri merited to become a king of Israel in reward for having added a city in Eretz Yisrael: Shomron. If the city had already existed, with Imri only fortifying it, the Gemara would not have stated that Imri “added” a city in Eretz Yisrael.
The Aruch L’ner defends the other Rishonim in a different manner. He argues that the four metzorahswere not halachically required to leave Shomron (since it wasn’t walled in the days of Yehoshua). However, the kings of Israel were often known for their jealousy of the kings of Yehuda and they therefore wanted to have their capital, Shomron, on par with Yerushalayim. Since Yerushalayim had exceptionally stringent laws of purity, they invented baseless restrictions for Shomron. And that’s why the metzorahs were exiled. (See also Meshech Chochma, Parshas Metzorah.)
Two Kinds of Metzorah
Rabbi Yehonasan Eibshitz explains that the metzorahshad to leave Shomron because they were not stricken with the tzara’as that the Torah discusses. Rather, they had a contagious skin disease, which the passuk compares to tzara’as. They were exiled from the city so that they would be quarantined – so that no one else in the city would be infected.
Rabbi Ephraim Margolios argues that the fact that the four metzorahsshared company with one another serves as a proof for Rabbi Yehonasan Eibshitz’s theory since a real metzorah must be isolated (Margolios Hayam 15, Sanhderin 107b).
The Rogatchover Gaon (Tzafnas Panei’ach, Tazria 13:46), however, disagrees. Citing much proof, he argues that although a metzorah is forbidden to associate with regular people, he may associate with other metzorahs.