Is this a balk move?

[ump24]

The comment was made that it is a balk to first because the RHP did not step off the back. I agree, to not step off the back isn't this move initially a feint to first base? The arguement was then made that a LHP balks in the same way by turning and throwing to second. WRONG!!!!
Once the LHP crosses the "midway" line he is committed to throw to second or to pitch the ball. He may no longer throw to first. THIS PLAY WITH A RHP IS A BALK. NO DOUBT ABOUT IT. THE ONLY PROBLEM IS WERE IS IT IN THE RULE BOOK?