I believe you're right. Broadcasters tend to focus on the positive rather than the negatives so they give more pats on the back to the winning team. I try to remember that when the Sox are losing.
However, that clown show on NESN is horrible - not because of their focusing on the winning team but because they seem to be playing to an audience that doesn't care as much about baseball as they do about all the peripheral issues like.... the quality of the food in other parks and how it compares to the park they're in, and how easy/difficult it is to get to the games.