Referendums are, by definition, the tool to change the law. In this case, to make legal something what was previously illegal.
Other than that, it makes sense that the Spanish law does not allow the Independency of a territory. I assume that the British law did not allow the Independence of the US neither.
I imagine that they'd have had different results if the referendum were about changing the law, but that's not what this was. This was specifically to violate the law.
Laws are intended to preserve convivence in society and secession is like a bomb put in the middle of the lifes, properties and projects of millions of people (Not necessarily for better. We have dozens of examples of how things can turn hairy easily for the population).
The argument can never be: must be allowed because currently is illegal, thus we must be lacking some good freedom here.
That's a solid argument.
The question is that in our democracy some themes are deemed illegal, and any web promoting them can be closed anytime and everytime, because this is how our laws work.
Spain has the fortune of being a melting pot of races and cultures in Europe. By our situation and history we have strong natural bonds with America, Europe and Africa. We have also suffered civil wars and dictatures and hate speech is seen as an specially disgusting thing by most spaniards. We know how this always end.
So, could be seen as "unjustifiable censorship" to close a web hypothetically hosting declarations like that?
The Andalusian has been hungry for hundreds of years and lives in a state of ignorance and cultural, spiritual and mental misery. (Jordi Pujol, ex-president of Catalonia and founder of the party currently in power, under a new name)
I have an clear opinion about that (this disgusting thing would be better closed for good ASAP), but let the judges clear if those webs were promoting hate speech and harassment towards more than half of the Catalonian people (not to mention the rest of spaniards), or not.
As for the voting? That's a bit more tricky. They are voting to commit an illegal act. While not morally similar, this would be legally similar to voting to allow women to be murdered for not obeying their husband.
Namely, they are voting to approve an act that is clearly illegal according to their constitution. I am not sure they'd be allowed to put that sort of thing on an officially sanctioned ballot, in the US. However, they could certainly hold a mock election, unaffiliated with the government, and it would only be of value for the optics.
I'm just pretty sure it couldn't be officially sanctioned.
This assumes the pundits are correct and that this is clearly illegal in their constitution. I haven't actually read their constitution and am trusting their information and interpretation.
In the US it's a bit stickier. While we've disallowed secession before, I don't think the constitution expressly forbids it. So, I couldn't quite come up with a similar example to match the US. In fact, I think we may have even agreed that Texas can seceede but it needs to be voted on with the same numbers as they would need to amend the constitution. I think... Don't quote me on that.