It's basic fundamentals, too. Screening a defender should be reliant on using the screen effectively by the guy who is trying to get free. If he leaves a gap or doesn't make a sharp cut, then he shouldn't get open. Allowing the screener to step into a defender promotes bad basketball and rewards laziness.
I blame Kevin Garnett. Dude never set a legal screen once in his career and I'm not sure he got called for it once either.