A few years ago in the UK the governing body stipulated that all promos and commercial be 16:9. I guess there's no such rule here as broadcasters continue to show a mixture in ad breaks.
I don't understand why someone would make a 4:3 spot - especially if it's a movie promo for a 16:9 film but there you are. I suspect it's just always the way it's been done and no-one ever said hey, let's make them 16:9.
I wish freeview would do it properly as then maybe I'd be able to watch TV content on iTunes. At the moment if I export it's super stretched.