kedushas arei chomah and machaneh yisrael

A naara ha'meurasa who engaged in znus is given skila on her father's doorstep.  However, if her father is no longer alive, in a city where the majority of inhabitants are Jewish, the punishment is carried out at the gates of the city; in a city where the majority of inhabitants are not-Jewish, the punishment is carried out on beis din's doorstep (Kesubos 45b).  

Tosfos asks: there is a din that the location of the beis ha'sekila must be outside the machaneh, which would mean outside the walls of the city.  How then can the naara be stoned on the doorstep of beis din?  Beis din sits in the city somewhere, not outside it!

Tosfos answers with a chiddush: a city which is majority aku"m no longer has the din of machaneh yisrael.  A metzora would not have to be sent outside its walls, and neither must the beis ha'sekila be located outside its walls.   

What's the basis for this idea?  Rashash suggests that Tos follows the view that when an aku"m has a kinyan in Eretz Yisrael it removes the kedushas ha'aretz.  

R' Shachter and R' Bakshi Doron each offer alternative explanations for the shitas haTos, the common denominator between them being that it has nothing to do with kinyan.  (Mishne la'Melech writes that the Rambam agrees with Tos din, but the Rambam does not hold that an aku"m's kinyan has an effect on kedushas ha'aretz, so there must be some other hesber.)  R' Shachter (p24 here) quotes a GR"A that cites a Yerushalmi that holds a city that is rov aku"m is like a ruin.  R' Bakshi Doron writes (seems to me that the language gets a bit muddled at the end of this teshuvah) that having a rov yisrael in a city is what endows it with the potential to be a machaneh yisrael if it has a wall.  

Either way, based on this Tos, R' Shachter and R' Bakshi Doron suggest a deeper meaning behind the gemara that says it is better to live in a city in Efretz Yisrael where there are a majority of aku"m than to live in chu"l in a city with a majority of frum residents (Kes 110).  One might have thought that the mitzvah of yishuv haaretz does not apply to a city of rov aku"m because there is no kedushas yisrael to that city anymore.  Kah mashma lan that the mitzvah of yishuv haaretz is not dependent on the kedusha of the land, but rather simply on its geographical location, i.e. the shem Eretz Yisrael, not the kedushas ha'aretz (as RYBS formulated the distinction -- see these posts I, II for more on that.)  

In the Oznayim laTorah, R' Sorotzkin is medayek in the language of the pasuk regarding nigei batim.  "V'kipeir al ha'bayis" (19:53) -- is it the house, an inanimate object, which needs kaparah?!  He explains that a Jewish home that has a mezuzah on the door, where Torah is learned, a home that is the center of a frum life, becomes a makom kadosh.  When there is a nega, that kedusha is defiled temporarily and needs to be restored.  

The same can be said about a city which has a rov yisrael living in it in Eretz Yisrael.  The religious life of its inhabitants makes the city a special place.