The Seahawks are "touchy", for sure. But a careful reading of the rules reveals part of the reason they get away with it. Most people assume you cant touch at all beyond 5 yards. But thats not true. You cant impede progress beyond 5 yards by touching. If your hand is one them but doesnt affect their movement, that it fine. And that is part of what they do. Its like the martial arts thing..keep your hand on them to help you react to what they do..in this case it seems to sometimes provide the clue to Sherman especially as to when to turn aroun.
The second half of the equation, in my opinion, is the Seahawks using press coverage. If your CBs line up 7 yards down the field, then their initial contact is going to be probably 8 to 10 yards down the field. Where it is already illegal. Obviously, blatantly. If you line up on the line of scrimmage, which the Seahawks do more than anyone else at this point, then your initial contact is right there. You have 5 yards to play with..and if it goes another 2 or 3 yards as you lighten up the touch.. then its just not going to be called. Its like when the speed limit is 60mph , youre just not going to get a ticket at 64mph , except in Modesto, California (a big screw you going out to Officer Wycoff!). If you get 5 yards to maul...youre gonna get away with 7.
So.. to me..those two factors go along way in explaining the lack of flags. But as others have noted, plenty of teams do what could be called PI every play. probably just as much. its just not as focused on because they dont play the press and they havent had the success so the spotlight isnt on them.