same reason China forbids or controls US companies operating in China. This is just tit-for-tat.

This isn't true, US companies are allowed to operate in China. They just eventually choose not to because complying with Chinese censorship laws is too much trouble, but in that sense they are not too different from Chinese companies. Facebook for example operated in China for many years until they decline to comply with a ruling on Xinjiang (which may have been the moral decision).