If this was a younger kid game, we all agree that we would have eased up. In fact, our local league does not record more than a +5 differential for any game so coaches do not feel the need to run up the score.
But this is the Freaking World Cup! These are the best 24 teams in the World, striving to be the best team, in the world. Goal differential does matter, getting your team to be in perfect form does matter, and gaining confidence does matter.
As has been pointed out, Thailand has been on the other end of similar scorelines in the last year, so they understand it comes with the territory. The US team was very kind and generous to them after the game. The only thing the US could have done to ease up would have been to play pass around, and that would have ben more humiliating to me if I were on the Thai team. An opponent deciding that they no longer have to try to beat you is the worst humiliation there is.
Thailand came to the World Cup to compete, they didn't come to be a sweet side story. When you are competing at the highest level you have to be ready to compete for 90 minutes, and they were not ready.
The only goal celebration I had any issue with was Rapinoe's, I have no idea what the leg kick thing was, but I don't understand her much of the time. Smiling and being happy you scored a goal, hugging teammates when scoring in a World Cup, those are all completely acceptable to me.