Before the NFL came up with defenseless receiver penalties, and helmet-to-helmet hits were not penalized and fined? NFL defenses were able to impose their will on the opposing team by striking fear in their opponents. Strong defenses still do impose their will, but are now limited as to what they can do.
The NFL came up with these new rules to protect high paid players and to provide legal protection in the event that players tried to sue the league for lingering affects of constant hard hitting.
The intimidation factor is not as prominent, thus giving the offense a more relaxed approach to calling plays.
Players like Ndamukong Suh and James Harrison have had to scale back their intensity and brutality to not get penalized for vicious hits. The NFL now intends to not only fine the players, but also punish the franchise financially and/or even strip them of draft picks for dirty plays.
Here is a video of NFL hits that were once legal:
Image courtesy of Jeffrey Bealle @ Flickr