A referendum conventionally happens country wide, surveying a % of the whole population. Not in a self-selected population subset.

Otherwise there's this icky question of what subset, what "social unit", is allowed to claim independence? Spain? Catalunya only? Barcelona only? La Rambla? Jose down the street, as an individual, in his own referendum-of-one?

This ad-absurdum may sound extreme, but there's really no fundamental difference there. The government astutely identified this as a "slippery slope", with brute power as the primary differentiator in practice.