I think the only time the public gets upset with an athlete's performance, that I've really noticed, is when there is a perception that s/he is failing to live up to innate potential by purposefully slacking off or squandering it. I think Bode Miller got hit with that rap in the last Olympics because he failed to perform to expectations and because he joked about partying at an Olympic level and that the combination of the two was absolutely key in defining that perception. It's just like when Tony Romo (has no one noticed that I've managed to bring up Tony Romo 2x and Brett Favre in the Winter Olympics thread now?) was partying in Cabo two years ago in the middle of the playoffs and the Cowboys lost as a strong favorite.
Even with that criticism, though, it's rough because who knows what the outcome would have been otherwise and then it becomes this built-in sound bite that defines you as an athlete until you defeat the slacker label. I was thinking that when I saw Bode being interviewed by Matt Lauer earlier in the week. The poor guy just won a gold medal and he looked crazy defensive. It really must take a lot of inner reserve to overcome all of that but, ultimately, if you do something perceived as slackerish it really can come back to bite you if you've already proven yourself as an athlete who has the ability to perform at a very high level. Other than that, I think people expect athletes do to the best they can.
Oh, and with the sportscasters, I think a large part of what they are doing is handicapping the event which happens with all sporting events. A lot of these winter sports aren't avidly followed other than during the Olympics and they are just giving the background the same way as when you might watch the Saints play the Lions and -- if you weren't aware (sorry Daddyzgirl!) -- the sportscasters will let you know that the Saints are the heavy favorites to win. It has nothing to with American nationalistic fervor and doesn't mean that the Saints are worthless if they don't live up to that expectation, but it's just an innate part of the sporting world to discuss it.