Also: the fact that you bought a metro tickets contains zero actionable information for any "agency". Great, you have a ticket. When will you use it, though?

Yes it does. It links my identity to the locations...

• I bought it
• I entered the metro system
• I exited the metro system

My travel through the city is now in a database. It's the sort of thing that a authoritarian state would find very useful to track down dissidents.

I'm not saying that Spain's current government would use it in this way, but they've constructed a system that would be very useful to a future bad actor. It's naive to think governments will always be benign and never abuse such systems.