Unwritten rules

I just read the book Calico Joe by John Grisham and part of the story is about "unwritten rules" in baseball.  For example, if a batter does something to show a pitcher up, he has the right to bean him.  Sounds like boys just wanting to belt each other with hard things to me. 

I've never played baseball, so I don't know all the unwritten rules.  I do know a slow pitch softball unwritten rule is that you don't hit the ball back at the pitcher.  It's bad form.  The thought behind this is that in slow pitch softball, the batter has more control over where the ball goes (if you have any sort of talent that is).  So here's my question, does the same rule apply for baseball?  Last night, Avila hit a ball right up the middle that managed to hit Duensing's left leg while in the air.  Then, as he put his left leg down to go get the ball, he fell over in pain.  It looked dreadful. 

This happens a lot though.  Ricky P is always sticking his hand out to try and stop balls as they whiz by his face (haha).  So, is it an unwritten rule, or is there not enough control when the ball travels over 90 MPH?