Tongue photos are split second photos and none of the Meghan photos are a rude sticking out. We put our tongues out briefly (and often unknowingly) all the time - concentration, nervousness, to show empathy or friendliness, to lick our lips, when we're hot or tired, opening our mouths a bit wider than normal...
If you look at photos of Kate it could seem like she has her mouth open wide all the time - huge grin, gurning even... Except if you watch videos, it's not the case, it's just a few seconds.