A vote for trade normalization with China is not a vote for China's government- it is a vote for the Chinese people.
Free trade is not aimed to benefit China's government, because trade occurs between individuals and between companies. It is to allow the Chinese greater participation in the international marketplace.
Furthermore, trade sanctions have proven ineffective in altering the behavior of foreign nations, and simply punish the people for their government's misdeeds.
If you think about it, trade sanctions from a powerful country such as the United States are a powerful propaganda boon to oppressive governments- not only can they claim that the US is evil, they can provide demonstrable evidence in that the US is working actively to avoid dealing with their industry, and thus advance the concept of pariah statehood.
It is obvious that China's entry into the WTO is assured- and that even without permanent normalization, it is likely that the US will enjoy similar benefits to the rest of the world, trade-wise. So, it then becomes arguable whether or not permanent normalization is necessary, if all benefits are already available under current treaties.