Pitchers hit batters for the slightest of infractions, most of which are legal in the rules of baseball. Some unwritten rules prohibit crowding the plate, sneaking a peak at the catcher's signs and position, hitting a HR, watching your HR leave the park, sliding too hard during a double play, breaking up a no hitter by bunting, etc...ratting is worse.

In this case, it's ARod against the league, not ARod against one pitcher. That's why you may see ARod get plunked in every series, maybe even during batting practice!