It has nothing really do to with that. Chinese government doesn't care about left wing American students and what they think. It is just doing what it can to promote its message, like it has been doing for decades.
the chinese government may not care what left wing american students have to say, but if those students establish a precedent that speakers deemed 'offensive' can be shut down solely for that reason, then the groundwork is laid for parties who _aren't_ left wing students to abuse that precedent as well. this is why we must defend the principle of free speech even when we disagree with the speakers.