Penalty after whistle

[#olderthanilook]

Scenario:

Gm 2 of the Blues/Kings qtr final in St Louis.

LAK player skates into the StL offensive zone off side. Whistle is blown. Blues player pushes Kings player to the ice. Official sends Blues player to the sin bin for 2 mins - cross check or tripping (can't remember which).

I've watched hockey all my life and am really into watching the officiating/rules side of the game moreso now than ever since I've been officiating hoops and football for so almost a decade.

Question I have is, why wouldn't the contact either be ignored or called for "roughing" since it occurred after the whistle, instead of the aformentioned "cross check" or "tripping" violation?