The part about playing the puck is relevant to Bischel, not to Cizikas. It’s illegal to hit someone who hasn’t touched the puck.
We can get into details of rules, but these guys have played their whole lives. And when there's a a free puck, and the paths of two pursuers intersect without either having position on the other, contact is a (unpenalized) reality of the sport. It's even a reality of non-contact sports like basketball and soccer, but it is the norm in contact sports.
I coach youth lacrosse, and one of the most important thing we do is use practice time to help players learn to protect themselves, particularly in these situations because they happen so often. It's actually much easier to teach it to kids in these situations (loose ball/puck) than it is self-protection with the ball (or puck), because some kids get totally single minded/tunnel vision when they have the ball (or puck).
We remind our players constantly, "chill out - they can't score from behind the goal line" for a couple of reasons, and positional awareness and spacing is obviously one reason, but self-protection is the other. Similar reasoning/exhortations apply to sidelines/boards.
It does not surprise me in the least when it's a 6'7" pro who doesn't have the habits/instincts etc. to keep his shoulder closed towards the extended goal line. It is very difficult for someone who is enormous relative to their peers their entire lives to develop the requisite anxiety to protect themselves if they don't come by it naturally. He got hit in the head by a 5'11" player who stayed level through the contact which occurred below the goal line, and came from the expected direction. If a much smaller player lacked such basic habits, they would never get close to playing pro.
There are times when players have no time to protect themselves, or are not in situations where they can know to, or it wouldn't make a difference if they did. This was none of those. This was a routine, 'take an angle towards the puck, don't open your shoulder, brace for contact, and if you get beaten & it rims around the boards, get back into position' situation.