LOL - Not sure what the ref's thought was on that. I've watched several times and I see a few things...
Letang doesn't trip him with his stick but with his skates.
Letang one handed swat at the puck actually makes contact with the puck, knocking it away, before any part of Letang makes contact with Couturier.
Letang never sticks his skate, leg or whatever out to deliberately trip him but when he turns his skates sideways to stop that is what gets Couturier.
So with all that said (especially with it appearing it wasn't intentional) a trip is a trip. It use to be if you made contact with the puck first it isn't a trip but, correct me if I am wrong, they changed that a few years back!
One thing that will never change in the NHL is .... poor officiating!