Question

Hello Rabbi Michael,
It is known that there are forms of impurity that the Sages decreed on their own initiative, not because of some concern about impurity, as they decreed impurity for hands (since hands are busy and touch many things) or the impurity of the lands of the nations (where people were not careful at all about impurity and purity). Rather, these are impurities that the Sages “invented” in order to restrict a certain kind of behavior, and therefore they decreed them.
An example is a Torah scroll, which renders the hands impure and in practice disqualifies terumah from purity-status use, because in their times they would place terumah together with the Torah scroll, and mice (or other animals) would come and gnaw at the Torah scroll as well. Similarly, the Sages enacted impurity for gentiles, that they should be treated like zavim, so that Jews would not be secluded with them, even though by Torah law a gentile as such does not impart impurity.
 
My question is: from where do the Sages derive the authority to decree this kind of impurity? On the face of it, even if one does not understand all the reasons behind the laws of impurity and purity, one can still see that there is a metaphysical-spiritual reality here that the Torah itself established and defined… So where did the Sages get the authority to come and “invent” new impurities that the Torah did not speak about? And one should further ask: why is there no issue here of “do not add” to what the Torah already established? After all, this is not to distance people from impurity, but for entirely different purposes.