Following up on the previous post, the Yerushalmi (which Rambam quotes l’halacha, Ma’aser Sheni 2:12-13) adds one additional important detail not mentioned yesterday. If ma’aser sheni became tamei either in or out of Yerushalayim from an av hatumah, or from a vlad hatumah (see yesterday’s post) outside Yerushalayim, the halacha is that we do not apply the din of “kaltuhu mechitzos” – you can be podesh the ma’aser sheni even in Yerushalayim and eat the food anywhere. (The exception noted yesterday is if it became tamei in Yerushalyim from a vlad hatumah, you must be podeh the ma’aser sheni, but the food must remain in Yerushalayim because of kaltuhu mechitzos). The Yerushalmi adds that if the food is tamei from a vlad hatumah, the owner of the food must make a tnai and stipulate that the din of kaltuhu mechitzos not apply upon his entrance to Yerushalayim, otherwise the default din of kaltuhu mechitzos will take effect.
R’ Yosef Engel in Esvan D’Oraysa (#10) cites this halacha as proof to the question of whether issurei derabbanan are issurei gavra or issurei cheftza. If the din of kaltuhu mechitzos does not apply to food which is tamei, why should whether or not the owner stipulated anything make a difference? However, if one assumes that food which is tamei m’derabbanan is not the same as a chefzta of food which is tamei min haTorah – tumah derabbanan is really tahor food that the chachamim impose restrictions on the gavra from using, then we can understand why an additional tnai is required to remove from this cheftza of tahor food the default din of kaltuhu mechitzos. R’ Yosef Engel brings a number of proofs in the rest of the essay to the opposite position, namely that issurei derabbanan are indeed issurei cheftza.