Why wouldn't it be, and who says it isn't? The subject is "cutting", a gerund serving as a singular noun, making the verb conjugate as third person singular. The full subject is the gerundial noun phrase "cutting onions", equivalent to "the cutting of onions".
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Cool BreezeYour request is a little odd. People usually ask why something is not correct.My thoughts exactly. One may as well present an entire volume written in English and ask why it's correct. What a job that would be.