The Ran explains (on Rif, Kiddushin 23a): "And now that it was stated this way and that way, all were based on the reason. Even though, in general, we follow the lenient opinion in Rabbinic matters, here, since it is not a matter of great effort, we choose the essential matter, as they explained. It seems to me that it is necessary to apply the reason in all cases, for if we were to follow the lenient opinion, why would we choose one over the other? And if we were to follow both, the reason for the mitzvah would be completely undermined."