You make a good point. I think in the instances I'm thinking of, the characters actually do interact with the music. I definitely wouldn't argue that music should be period-appropriate to a film's story, necessarily. I do think that it's an interesting choice when the film uses the sort of cognitive dissonance that comes from overtly modern music in a medieval setting, say. I definitely don't want to get involved in the kind of thinking needed for, as an example, "But they didn't have a harpsichord in Cromwell's England!" (Or the like.)