The Rishonim ask: The Gemora in Shavuos (36a) states that Rabbi Meir only holds that “the positive cannot be implied from the negative statement” only in regards to monetary matters; however, in regards to prohibitory matters, Rabbi Meir agrees that we may infer the positive from the negative. If so, the Mishna here can very well be following Rabbi Meir’s opinion, for we are discussing the laws pertaining to vows and this is not a monetary matter, but rather, a prohibitory matter?
Tosfos answers: It is evident from the Gemora there that a prohibition that involves money has the status of a monetary matter and Rabbi Meir will still maintain that “from the implication of a negative, we cannot hear the positive.” Since a vow involves money, for one is prohibiting the possession’s of his fellow on himself, Rabbi Meir would not concede in this case.
The Ran answers similarly that since by a vow, one is prohibiting the object upon himself, it is considered a prohibition that involves money.
(A difference between Tosfos and the Ran may be by an oath, where, according to Tosfos, it still would involve money because he is prohibiting himself from deriving benefit from his fellow’s possessions; however, according to the Ran, it would not be regarded as a monetary matter since the object itself is not forbidden.)
Tosfos in Shavuos answers that Rabbi Meir concedes only by a strict prohibition, i.e. one that a person will be liable to death; however, by a vow, which is not regarded as a strict prohibition, Rabbi Meir will still maintain that “from the implication of a negative, we cannot hear the positive.” (The Rashba does not agree that the Gemora states such a distinction.)
The Rosh answers: Since by a vow, we go according to the language of people, it is regarded as a monetary matter. Rabbeinu Avraham min Hahar explains: People generally explain themselves completely. Therefore, by a vow and all money matters (for the same reason), Rabbi Meir maintains that “from the implication of a negative, we cannot hear the positive.”