No they're not, the Bespin escape clip for instance is a 720p TS stream with no black bars and it plays perfectly on my widescreen monitor. I've noticed that some of the Facebook clips are stretched, but I think that's because of some other reason. If you play a 720p or 1080p clip with no black bars on a widescreen monitor it will create the bars itself.

But if the bars are already part of the image it will still create them and thus you have two sets of black bars surrounding your image, which makes the image look even smaller.