Can you hit behind the head if opponent bent over?

[Leighton]

Behind the head whilst standing is a flagrant foul. But I have seen countless scenarios where 1 fighter is crouched down so low where his head is about even with the other fighter's thighs, and he gets slammed square in the back of the skull with a downward punch. If he keeps his goddam head way down there like that, is it permitted that the other fighter can chop down right onto the back of his skull? It NEVER gets a warning as far as I have seen. He is giving no other target than the back of his head.

This reminds me of Sergio Martinez vs Kermit Cintron. Not sure about the deduction. The other guy was ducking when Sergio threw the punch. You be the judge.

I hate to see fighters using the back of there head as a defence mechanism. They also cant be hit when they turn around. I would say if you break the rule protect yourself at all times which this clearly is, then its fair game, but if that was the case people would be getting punched in the back of the head all the time and it would be a very dangerous sport.
I would like to see a more point deductions for putting your head down all the time. A couple of warnings from the ref and then a point taken.