The discriminant between the two positions is that 'free speech' (as constructed here, I probably disagree with the conclusion) need not be a binary. Where backdooring encryption is clearly a binary policy that once triggered can't be revoked, this is only because it pertains to hidden information. Mitigating the proliferation of specific classes of content can be done with specific intent because you know what types of content you're mitigating - fundamentally this is no different than a website (like this one) deciding not to contend with certain types of content (as the rules imply). This means that you break out of the 'slippery slope' argument pretty easily and can get access to 'not negotiating with domestic terrorists' without the negative externalities of suppressing open discourse.