Earlier quoted context omitted.
Can I mention a few counterexamples? I guess no need to because you know. Yeah in 2017 some things are no longer accepted in the heart of Europe but Yugoslavia, Ukraine etc are recent examples. IIRC, you have the right to leave in peace, use your language, autonomy and not be discriminated but declaring independence is quite a few steps above that.
However, freedom of speech is an inalienable human right (and in need of most protection when it's political speech) - so regardless of whether you think that government's power derives from the people, they should be allowed to talk about the referendum.
You can't say there isn't freedom of speech in Spain when even people convicted for terrorism speaks in public media everyday.