From the IIHF site
530 - High Sticking
a) A player who carries or holds his stick or any part of it above the height of his shoulders that makes contact with an opponent shall be assessed, at the discretion of the Referee, a:
-->Minor penalty (2')
or
-->Major penalty + Automatic Game Misconduct (5'+GM)
or
-->Match penalty (MP)
b) A player who carries or holds his stick or any part of it above the height of his shoulders that makes contact with an opponent and causes an injury with his stick or any part of it to an opponent shall be assessed, at the discretion of the Referee, a:
-->Major penalty + Automatic Game Misconduct (5'+GM)
or
-->Match penalty (MP)
c) However, if the high sticking action that caused the injury was judged accidental, the offending player shall be assessed a:
-->Double Minor penalty (2'+2')
I highlighted the seemingly relevant portions. Now, it was clearly accidental, so at worst it should have been 4 minutes. Where it gets hazy is in part b), where it does not define what exactly constitutes an injury and seemingly leaves that up to the discretion of the ref.
If that play had happened in regulation, we probably would have seen a double-minor called. I get the feeling the ref was not willing to give one team such an advantage in OT. I think it's a case of following the letter of the law but not the spirit.