From my perspective this is not a slippery slope argument as the violation of our rights are already taking place. Combined government and corporate power are already censoring legal speech. It is just more difficult to oppose because I mostly disagree with and find odious the speech that is being censored.

I think that not opposing this government censorship is a mistake and not just because of the principle.

While I do think the principle of freedom of speech, press, assembly etc are important principles, it is also true that censorship will tend to favor the right in their political aims more than it will help the left. The right has corporate and institutional power at their disposal and it is used to defend the status quo. It is the left that needs freedom of speech to disseminate their ideas. The right can simply rely on the existing power structures to maintain their ideological hegemony. It is the left must agitate and convince others that there are better ways of doing things.