There is history between the two players, Pacioretty taunted Chara after scoring and pissed him quite a lot in the last game between the two teams. Definitely a possibility that he picked this moment to get back at Max.
As for the league's ruling, it is a joke, as always. This league is a joke. They're not even trying to protect their players... including their top stars in fear of "denaturalising the game." This league is run by goons at every level, including the terribly awful players association that refuses to protect its own members. I have no respect for that association.
This league rationalizes injuries coming off cheap shots as 'part of the game' and it pisses me the fuck off.. especially when the reaction is the same as they lose their best players. It isn't as bad as it used to be (see Scott Stevens, a HoF career made off blind hits) but there's a lot of room left for improvement.
Although unrelated to hits, I never understood why players aren't required to wear full plate visors or a grid. Also, fights, while they entertain me, need to go.