It was only called because it was dangerous looking. I don't even disagree with the call, but the reality was he was 100% down before he made contact with the boards, and that contact was a light kiss by the top of his helmet. Call it for being dangerous but it's still frustrating to know that there wasn't even substantial contact with the boards.
Giroux clearly tried to take out Thornton's feet.
Because of how light it was, it can easily be argued he tapped to distract, and then the guy full out flopped. The crosscheck G took and didn't flop on (as criticized by JJ) had 5 times as much force behind it.