That's an interesting though. For all the shock and awe the apparent absurdity of the "pay on the spot" laws seem to inspire on folks, I can't really think of a better system in this context.
Sure, it looks damn shady, and if it happened in Brazil I would be totally suspicious of the cop's story. But would pay anyway, as part of a cunning "not getting shot in the face" strategy.
But it actually looks like a decent system providing it's a place where 1) people generally take the laws seriously, and 2) the cops generally aren't criminals. I would guess that it actually works fine in Spain, since from my knowledge western europe is not exactly a wretched hive of scum and villainy. And what else can they do, considering a bus over there can be three countries away in the next day?
Quick question for our knowledgeable USAsian friends: What would happen if the same situation happened in the U.S.? I mean, suppose a foreign non-U.S. resident commits a fineable traffic infraction inside the country?
- Pay on the spot?

- Do the cops say "No U.S. address? Then I'm sorry for the inconvenience sir, we cannot fine you, have a nice day" and let you go?
- Do they FedEx the fine to your international address and get the Interpol to arrest you if you don't pay?
- Instant deportation and blacklisting so you'll never cause trouble in american soil again?
- Guantanamo?

Seriously, what happens?