some interesting parameters of gezeiros

Among the differences between a kohein and kohenes listed by the Mishna in Sotah is the fact that only a kohein is prohibited from becoming tamei meis but not a kohenes. The Yerushalmi (Sotah 18a in Vilna ed) quotes Rabbi Dosa as saying a chiddush that a kohenes is also not included in the gezeirah not to leave Eretz Yisrael to go to other lands (eretz ha'amim).

The Yerushalmi asks why R' Dosa's chiddush is needed. The whole reason for the gezeirah not to enter eretz ha'amim is because of potential tumas meis. If a kohenes is not prohibited from becoming tamei meis, obviously the gezeirah is moot.

The Yerushalmi answers that one might have thought that since women are generally bound by gezeiros, here too, even though the reason does not apply, one mighty assume the gezeirah is still binding.

Why don't we accept such a sevara? The Yerushalmi answers, "nimtzah matche parshas tumos" -- obligating women to observe the gezeirah would undermine the pasuk's explicit exemption from the laws of tumas meis.

A few thoughts on the give and take of the gemara:

The idea that a gezeirah can be binding even if the underlying reason is inapplicable which the hava amina highlights is found in other areas as well. For example, R' Elchanan Wasserman points out that according to the Ran (Nedarim 52) the reason a davar she'yesh lo matirin is not bateil is because there is insuffcient contrast between the potential-heter item to be bateil and the mixture of real heter which it falls into. However, the application of the din extends beyond the world of ta'aroves to encompass any situation of safeik which will ultimately come to resolve itself (see Beitzah 3).

The conclusion of the Yerushalmi does not (I think) seem to retract this premis, but instead follows a line of reasoning popularly attributed to the Taz (Y.D. 117). The Taz writes that although Chazal have the power to extend prohibitions by creating gezeiros, no gezeirah can encroach on what the Torah itself explicitely permits. For example, Chazal created a gezeirah to prohibit blowing shofar on Shabbos. There is no such similar gezeirah prohibiting milah on Shabbos. Why the difference? Rishonim explain that the Torah explicitely commands that milah be done even on Shabbos; no such explicit command exists with respect to shofar (see Gilyon Maharasha to Y.D. 117 for other examples). It sounds like the Taz's chiddush is anticipated by this Yerushalmi: "Nimteis madche parshas tumas" implies that a gezeirah cannot infringe on the explicit permissability for women to become tamei set out by the pasuk.