That's only by custom. Legally the photographer holds the copyright as soon as they take the photo, but they could just as easily assign full copyright to the person hiring them as a matter of course.

Most photos of people at weddings, family portraits, etc have little commercial value, except to the people who are in the photos. So you can see how this feels like double/triple/n-times charging, as it has suddenly become a monopoly backed by federal copyright law.