Make no mistake: this is a form of taxation. Our (i'm spanish) minister of economy has been very imaginative in the last 4 years to find new sources of revenue.
As just one of many arguments against: Such decisions are subject to judicial reviews (i. e. Facebook can sue). Spanish judges are just as independent from the executive as they are in most countries, once they are appointed. What motivation would these judges have to undermine the rule of law (something they studied and have worked in their whole life), when they can't be pressured, nor are they recipients of any part of such fines?
It gets even harder to think of a possible motivation when you realise that any court rulings could be appealed in European courts, completely out of the sphere of influence of the Spanish government.
The EU countries are playing this game since the crisis started in 2008. Spanish politicians just want their share. All of the Ireland-Google issue was exactly the same but bigger: even if they abide the law, politicians tweak a new one just to milk big companies. They just take the money from where it is.
I don't know about conspiracies you talk. This is all happening in day light and applauded by the people.