The rules for positioning a parenthetical citation next to a final period seem different with run-in quotations and block quotations. What is the logic here?

Run-in quotations and block quotations follow the same logic, although the differences in their formats call for differences in punctuation. First, let’s look at a run-in quotation:

Virginia Woolf describes the scene vividly: “Everything had come to a standstill. The throb of the motor engines sounded like a pulse irregularly drumming through an entire body” (14).

The writer’s sentence begins with “Virginia Woolf” and ends with the citation, “(14).” The citation refers to the quotation and thus belongs in the same sentence with it. A period is needed after the citation to indicate where the writer’s sentence ends. In the source work by Virginia Woolf, there is a period after “body,” but it’s omitted here because the following period makes a period after “body” redundant.

Now let’s consider a block quotation:

Virginia Woolf describes the scene vividly:

Everything had come to a standstill. The throb of the motor engines sounded like a pulse irregularly drumming through an entire body. The sun became extraordinarily hot because the motor car had stopped outside Mulberry’s shop window; old ladies on the tops of omnibuses spread their black parasols; here a green, here a red parasol opened with a little pop. Mrs. Dalloway, coming to the window with her arms full of sweet peas, looked out with her little pink face pursed in enquiry. (14)

Here, as above, the writer’s sentence begins with “Virginia Woolf” and ends with the citation. In the block-quotation format, however, no period after the citation is necessary: the reader knows that the writer’s thought ends at the citation because a block quotation is not normally inserted in the middle of a sentence.

The period after “enquiry” is not the writer’s final period. It is Virginia Woolf’s period, found in the source work. It is retained in this format because there is no following period to make it redundant.

The two examples present the same sentence (except for the contents of the quotations). But the examples have different formats, which call for different periods to be dropped.