WeChat is censoring the messages, not government, they just give them order and WeChat gladly accept terms in exzchange for China monopoly and protection from international competitors, same go for Facebook, Google and million other services, which Chinese just stole and rebranded. So how exactly it's not their fault they are sleeping in bed with Chinese government?

But could it be part of the reasons for its success ?

Yeah, that's a great question. The reason they have the luxury of not caring about their user count could be that the government takes care of that issue for them by interfering with their competitors.