When R. Dimi came (from Palestine to Babylonia) he stated in the name of R. Johanan: On a place (between a public and private domain) whose area is less than four handbreadths by four (i.e. too small to be it’s own domain) it is permissible both for the people of the public domain and for those of the private domain to re-arrange their burdens, provided they do not exchange them?
The problem with “exchanging” is that forget and assume that one could carry from private to public. It is therefore a kind of “fence around the Torah” and rabbinic enactment meant to protect basic Torah laws.
and the Sages have applied to their enactments, heavier restrictions than to those of the Torah.
An interesting observation that the rabbinic laws can be more strict than the Torah laws!