From reading on Justine Sacco, she made a very poor decision in making such a remark for the very public platform of Twitter.
How is the world supposed to assume she was joking and why does it being a joke validate making such a terrible comment?
Is this any different than anyone making a poor choice and having to deal with the consequences of it?
Being someone employed in PR, how did she not know the ramifications of such comments being put out there for the world to take as it did?
Would you not expect a man in PR to be fired if he had made a sexist joke on Twitter?
She cannot control how the world reacts to her. She can only control how she reacts to the world.