It's not about being PC or not, it's about conventions that exists and are not the same for all productions. If you take a movie like, say, Mel Gibson's Apocalypto, there is a great work done to create a feeling of realism. You hire mexican actors of mayan decent, you carefully recreate clothing and textiles based on archaeological studies, you have the actor talk in what we believe is something similar to language used in the time and place the movie depicts, etc. But when you take something like Joe Wright's Anna Karenina, the source material is a russian play set in Russia, yet, the film is spoken in english with a cast of non russian actors and there is a tremendous work done with the sets and the film making to play with the notion of make-believe (a movie based on a play about people in an upper class where appearances are more important than reality). Movies run a broad gamut going from the very "realistic" to the very "artificial" and there is probably a bigger tradition in theater and in opera to have things lean toward the latter. Typically, musicals are among the most "artificial" movies there is and, as such, there is a number of conventions in play, a number of tacit agreements between the spectators and the film makers. People will start singing and dancing and that's how parts of the story will be told, that's one of the many conventions, that's supposed to be accepted by the spectators when they enter the movie theater. The only thing I am saying is that, apparently, in Beauty and the Beast they add another convention that is routinely accepted by theater, opera or ballet spectators: any ethnicity can play any part.
Obviously, for something to become a convention, it takes familiarity and familiarity takes time. So, at this juncture, it's only natural for people who are not familiar with this convention (as it comes from other art forms) to find it jarring. Just like the singing for someone who has never seen a musical or a reverse angle for someone who has never seen a movie.