The step back pitching move is legal in high school play, but not legal for other organizations. (Well, it is legal for adult men's pitching, which doesn't really apply here).
The "sense" of it might be debatable. Years ago (around the mid-1980's), the step back was legal for ASA and others. As pitching became more dominant, the rule was tweaked to disallow the step back, the thought being that if the pitcher was creating a little bit less momentum it would give a little bit of an advantage back to the hitters.
The rules for high school ball stayed the same as they always were, allowing the step back. The high school rules makers decided that pitching in the high school game didn't need to be throttled back and that the less-dominant high school pitchers should have that one small advantage.
Other sanctioning bodies gradually changed their rules to eliminate the step back. In fact, it was legal in USSSA ball up until just a couple of years ago.
Besides high school ball, there might be a couple of others that still allow the step back. I believe that Little League is one of them that still does. But as far as "the big three"- ASA, NSA and USSSA- none of them do.
The strange thing is that most people consider the high school rule to be the odd one, when it is the one that hasn't changed over the years. All of the others did!
