The lenses project the image plane such that your eyes focus as if they were looking at an object about a meter away. It doesn't cause the kind of eye strain you would get from trying to focus on something an inch in front of your face; most people's eyes can't focus that closely to begin with.
It does cause the kind of strain that looking into a light bulb might cause, not considering the focal point. But this is probably similar to using a desktop monitor.
