Here's the problem with what these DB's and Safeties keep saying, they're basically being selective about the rule and only mentioning what they want. As if its an excuse to target a guys knee. Here's the defenseless player rule, I've highlighted the parts that apply to receivers and this hit.
Basically, there are two points. Gronk was on his third step from control of the ball, if he's not out of the defenseless receiver protection, he's very close. So hits anywhere would be open and you can treat him like a running back. So leaving your feet to go at his knee was a choice, not something you did because you were worried about a fine. T
hey're targeting the knees and using the rules as an excuse.
Second, if he was still considered defenseless, you're not allowed to leave your feet and launch at any part of his body provided the launch is outward and upward. So they launch at the knees because its downward. Justifying it by saying they have no other place to tackle, which is bullshit. He left his feet and launched his helmet toward the front of Gronk's knee. Claiming Gronk is big and he can't tackle him any other way. As if we didn't notice the linebacker simultaneously wrapping him up and a normal tackle coupled with the LB wouldn't have brought him to the ground.
Personally, I'm not sure if Gronk was out of the defenseless receiver zone.
Watching the play, It looked like he was starting to protect himself from a high hit, so he was beginning an act of warding off impending contact. If he's no longer defenseless, his head is in play and Ward is allowed to hit him all the ways he's complaining that he can't.