Indeed, when it says several times (in the Talmud) מדרבנן וקרא אסמכתא בעלמא (it is by rabbinic law and the verse is cited as a mere support), it is an interpretation, not a proof from the language nor from a basic rule, and the sages rely upon it in a distant way. And it was clarified by us that it was this way because sages of blessed memory did not have the power to decree or to establish a matter that is the opposite of what comes explicitly in the Torah. And when the adjucators of Jewish Law wanted to issue their decrees, they were forced to prove that there is nothing explicitly in the Torah that is the opposite of their decrees and that the Written Law tolerates another interpretation that is in agreement with their decrees.

See Rule 235 for a definition of אסמכתא.