I think that overall the EU wants peace amongst the nations and all of the world. So a ban on arms sales is very consistent with the European Union's strategic objectives. It ensures that countries and people are not selling arms to nations that are violent. This is consistent with their peace strategy.
China has no difficulty securing the weapons they want. They are an incredibly rich country with a strong military. The EU ban on arms sales has no practical effect on China. But, it is an important symbolic gesture that shows that the EU does not agree with the repressive acts of the Chinese government. This ban shows to the world that the EU will not supply weapons to a powerful an dtyrannical regime.