Well, there are two answers to that good question - because paying attention to subject movement (as well as camera movement) is very important in HDR photography.
1 - Subject movement can be good - when you are creating HDR images of fast-moving water and clouds.
Even exposures of several seconds can help to create beautiful images, as illustrated here by my HDR image of the New Croton Dam.
2 - Subject movement can be bad - when photographing people. For example, check out the close-up of the man's hand in the top set of illustrations (San Miguel de Allende, Mexico). Even though I asked him to stand perfectly still, he still moved his hand ever so slightly in a few sets of images.
When photographing people, my advice to HDR photographers to take several sets of pictures - with the goal of getting at least one set with no subject movement. Plus, of course, asking the subject to stand perfectly still.
You'll find more info on HDR imaging in the next update (late March) of my 24/7 Photo Buffet app and in my next book (due out in late April), HDR Photography Secrets.
Explore the Light,
Rick