http://www.boston.com/sports/football/patriots/reiss_pieces/2008/09/bill_belichick_8.html
The part in bold says it all. I'm sure the league would agree with that statement. So in saying the hit was legal (and nothing else about it that I could find), the league is saying that Pollard acted responsibly and only through Morris' actions was he directed into Brady's knee. This only makes sense in 2 cases:
1) Morris blocked Pollard in the direction of Brady (didn't happen)
2) Pollard was aiming to hit Brady above the knee but Morris caused him to hit lower (didn't happen)
If the league doesn't agree with Belichick's statement in bold, under what conditions is a player not responsible for slamming into a QB's knee (besides the 2 I listed above)? Getting knocked to the ground near a QB? Having a blocker near you while you are attacking the QB? Both of these situations happen all the time and I don't recall players using the opportunity to cripple an opposing QB.
BTW, I wouldn't have had an issue with Pollard reaching over and grabbing Brady's leg...even if that resulted in the same exact injury. Turning your helmet into a missile and launching yourself into Brady's knee? Not so much.