Uniform spacing around punctuation also saves a step in preparing word-processing files for electronic editing. As a publisher, APA does not return manuscripts on the basis of the spacing around punctuation. A couple of guidelines in chapter 2's Guidelines to Reduce Bias in Language are at work here. One is to acknowledge human participation. The other is to be specific.
Subjects is a fairly nondescriptive, passive term.
Identifying human subjects as participants, respondents, children, patients, clients, and so forth increases specificity.