Catalonia is essentially Spain's Northern Ireland.
Except without a body of water separating them and without a country right next door that they'd probably join should they gain independence.
But anyway, WTF Spain? The courts said that the referendum had no legal status, so where's the fucking harm in letting them vote? Seriously, this looks way worse than letting them vote and then ignoring the result--especially if, as a lot of people expected, the vote was boycotted by those opposed to Catalan independence.