Do as you are told, peasent!
Precisely the point mate; everyone has the right to hold a referendum on membership of a region or union. The Spanish Government can argue the point against holding one, but to be seen to use force to prevent it? As someone else stated the whole world is watching this; just because Catalonia may vote for independence it still does not grant them the power to have it. In essence the exercise is merely a 'statement of intent' that would mean the Spanish Government having to address the issue rather than bury their heads in the sand. Bleating 'it's unconstitutional' means nothing as they can be re-written, changed, adapted. Spain could lose a lot of international support by not addressing the Catalonians as we in the UK did with the Scottish Referendum.