Under G.H. Bush (Bush sr. to some) China conceded to "capitalist zones" in their country to keep their trade status with the US. This allowed foreign/private ownership of economic instruments and is the key to their current economic "success".
We shouldn't do business with them because instead of promoting socialism as something that works it encourages the spread. By us doing business with them, we would be giving false data on how socialism really works. Just like China was an economic/social failure prior to capitalist zones being established, any country choosing socialism should suffer the depredations of socialism without receiving any benefits from those who practice some form capitalism.
Further, socialism being an ideal which ends in theft and slavery is the enemy of freedom. If you believe in freedom, then socialism is your enemy and any business done with socialist only provides benefits to your enemy. That is why I don't even buy products that I know are built in union shops or come from liberal states.
Fear has nothing to do with it. An enemy is an enemy and should not be given aid, comfort or any kind of help but should be encouraged to wither and die on their own if possible or destroyed militarily if necessary.