They changed it in the US so that it would sound more "magical" to the kiddies. I have no idea why this is, really. Sorcerer's Stone sounds no less magical than Philosopher's Stone. And hell, since the Philosopher's Stone is supposed to be a true artifact of legend, it just says to me that Rowling but research into what she was doing. Calling it a Sorcerer's Stone kinda takes away from that, methinks.