According to the basic halacha, a meis mitzvah must be buried wherever his body is found, unless the burial would be an issue for kohanim who pass by that area. In practice, though, this rule is no longer followed; the body is transferred to a cemetery instead. The poskim explain that outside of Eretz Yisroel it is not feasible to bury a meis mitzvah in his location, since the government will not permit a burial outside of a cemetery. There is also a concern that non-Jews may uncover the grave and degrade the body. Regarding Eretz Yisroel, contemporary poskim say that the body should be transferred to a cemetery because there is no central authority available to certify places where tumah may be an issue.
[ביאורים ומוספים דרשות תקכו, 42 ;ב"ק פא, א]