Why is it illegal? Let's go back to 2005 for the reason.
''QUARTERBACK HITS ... The hit that resulted in Carson Palmer tearing his anterior cruciate and medial collateral ligaments is also coming under scrutiny, and there is some feeling among competition committee members that all hits below the knee on quarterbacks should be penalized.''The rule that came to pass in 2006;
''A rushing defensive player won't be allowed to forcibly hit a quarterback below the knees. He has to make every effort to avoid such a low hit.''
It would appear a rushing defensive player completely got off the hook yesterday.