CORRECT POSTURE, POOR POSTURE

Right is the correct posture whose spine forms natural S-curve (A), pelvis tilts forward (B) and knee isn't pushed forward (B). Left is the inverse of it. These are some important points of  difference between correct posture and poor posture.

These hitters' batting stance has the problems that I described above. It is necessary to note that those problems potentially cause some diseases related to knee joint or lower back. 

Bryce harper is at the risk of knee or back injury.

Jeff Bagwell's batting stance had fulfilled those rules and that is why he succeeded in spite of such a tough batting stance.