When a comma or period is needed after a quotation, publishers in the United States typically put the punctuation mark before the closing quotation mark. The reason for this convention is to improve the appearance of the text. The convention goes back at least to the nineteenth century. John Wilson’s A Treatise on English Punctuation (1850) says that it provides for “neatness” (114). A comma or period that follows a closing double quotation mark hangs off by itself and creates a gap in the line of text (since the space over the comma or period combines with the following word space).
British publishers tend to put the comma or period after the quotation mark. But the British usually use the narrower, single quotation mark as the primary quotation mark. There is less of an aesthetic penalty to placing a comma or period after a single quotation mark, since the mark of punctuation isn’t stranded as far from the previous word.
It’s true that the convention followed in the United States treats the comma or period as if it were part of the quoted material. But the practice is “not likely to give a false meaning to the words cited” (Wilson 114). Indeed, this sleight of hand involving punctuation is minor compared with the violence of quotation itself: quoting almost always entails wrenching the original author’s words out of their context, an action that inevitably affects their meaning.
The conventions of scholarly quotation—removing the original context, adapting the extract to fit the new context, and others—are well understood. They don’t unduly compromise the source if they’re followed carefully.
Wilson, John. A Treatise on English Punctuation. 2nd ed., Boston, 1850. HathiTrust Digital Library, 13 May 2012, hdl.handle.net/2027/hvd.hx521x.