Backing away CAREFULLY is how you get safely away. In 4E, that's a shift. In 3E, that's a 5-foot step. If you just bolt from a melee at normal speed you'll get clocked.
I agree with this, but it does highlight a failure in the rules in that is imposable to withdraw from combat safely in D&D. Which is not actually true in real life at all.
Getting a 'free chop' on people withdrawing disorderly because they have failed a moral check I can totally understand, but just moving out of combat at speed should not present such an opening.