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.
Maybe it’s just my American perspective, but I’m a bit uncomfortable with this whole idea, voluntary or not.
The state who collects for X has the power to coerce X.