I think there are a couple reasons Kristen Stewart got the worst of it in her situation. While the director might have more clout in Hollywood circles (I have no idea), she is the one who is known to to the average person. Additionally, a lot of fans of the Twilight series were very invested in her relationship and maybe even mistook the real people for their fictional ideals. When she didn't follow the novelized version of herself people felt betrayed. The director did not have that sort of preconception out there so when he behaved badlly it was just him, not a beloved character.
As far as the Jason/Brittany situation, I don't know. They are both singers, but she was on American Idol, yes? Once again people might feel more invested in her, thus more disappointed. I don't know if that is part of what is going on but it might be. People look at ehr and think "I voted for her, I was rooting for her, I thought she deserved good things - and then she acts this way?!"
I do think women probably get more blame than men and to me that is actually a backhanded insult to men as a group. As if they can't possibly fight off any female they get too close to because they are men and can't be counted on to show self-control or moral fiber.
As far as being publicly embarrassed - that is tricky. Obviously threats are not to be tolerated, name calling is rude. On the other hand, when you are in a profession that depends on publicity and you do something that a lot of people find morally questionable then it seems a bit absurd to complain about public embarrassment whether you are an actor, politician, musician, etc.
The thing that gets me is that in some cases it is a big scandal. In other cases it is discussed as if the celeb in question is starring in a brand new love story.