Kate and William have six events on their trip to Boston, one of which is attending a Celtic baseball game. Every time their faces were shown on the Jumbotron ( giant TV screen) they were booed and there were chants of USA. William and Kate reportedly looked very uncomfortable at the game.
They hadn't done anything wrong at the game. They turned up suitably dressed and watched the game, that was all.
But I think going to Boston and especially to the Celtics game was always a mistake. They seem to operate on the basis that everyone will be thrilled to see the Royal Family and do not appear to understand that this is not always the case. They need to choose the places they visit much more carefully and drop any outdated beliefs that they will be welcome everywhere they go.