This is common European practice.
If the Spanish state were to seize the taxes that Spaniards pay on the understanding that they are contributing to the Church, then the Church's various charitable projects would be in serious danger.
This move would amount to a nationalization and subsequent secularization of religious charities.
Sounds a little like our own "do you wish to donate $3 to the presidential election campaign fund".
No thanks.
Send it to the Catholic Church instead. A far better destination.