The top point in support right now is, "By withdrawing sponsorship, private enterprise can send a message faster than governments that they will not tolerate the human rights abuse."
Companies withdrawing sponsorship of the Olympics would not show that they don't tolerate human rights abuses, just that they respond to pressure. They abet abuse other ways.
The Chinese government is more likely to listen to the withdrawal of capital before the rhetoric of politicians. If the money leaves, foreign governments are likely to follow suit
The Chinese government has not used the Beijing 2008 Olympics not as an opportunity to clean up their act. If they can get away with things now with the eyes of the world on them, they'll be emboldened.