Nazir 55 - Nazir outside of Israel?

In order to discourage people form leaving the Land of Israel, the Sages decreed that anyone who leaves Israel would immediately become ritually impure. Ostensibly, this is because people who live outside of Israel are not as careful marking off graves (ground-based impurity), and one can pass over an unknown grave. However, maybe the Sages simply declared the airspace outside of Israel impure (air-based impurity).

Perhaps the laws of nazir will shed some light on this? A nazir who went outside of Israel becomes ritually impure and will have to continue his days after his purifies himself. We also saw that he purifies himself with the ashes of Red Heifer. Now, the ashes are only needed for the impurity of the dead. This proves that the out-of-Israel impurity was because of the unmarked graves, and it is a ground-based impurity, does it not? - Actually, no. It could be that this is sheer air-based impurity, and the ashes of the Red Heifer are mentioned only in regard to a plowed-over cemetery , but not to anything else.

Art: Graveyard under Snow by Caspar David Friedrich