This sounds like some mental gymnastics to be honest. If you start to do that kind of reasoning, you might also say the police just came to get the ballot boxes, so they didn't stop anyone from voting.
The difference is that the spanish police didn't want to take the boxes, in fact the catalan police stopped more voting points than the spanish. The spanish police want to hurt people.
Doesn't matter if a generic "referendum" is legal or not. Having one for independence can be illegal. Using a gun can be legal or illegal depending on how it's used, for example.
Thus independence referendum was illegal.
We had already one referendum not long ago, Spanish didn't care at all and nothing happened, people who want to vote voted, and the central goverment didn't credit to it.
As a private citizen you can do that, you can't use public funds or entities for it though. Administrations are only allowed to call for votes over things that they can decide by themselves.
As the catalans we can't decide from ourselves? You mean we can't decide from things that aren't legal, but we are back to the origin, we should be allowed to vote everything.