Consider this setting. The red car would be in the wrong because he is in the left lane and tries to exit the duel carriageway. This is wrong in the UK and Spain.
Also the white car does not indicate to say it is not leaving the road.
Now Ignore the carriageway going the opposite way and crate a one way system that comes back on yourself and you have this,
which sometime does occur in the UK and Spain. Here also the red car is in the wrong.
From here it is a short step to see the logic for the Spanish insistence on driving all the way around a roundabout with no indicators.
So the way Spanish people are taught to drive and how the law in Spain expects drivers to treat a roundabout is like this.
If you are exiting the roundabout you must be in the right hand lane. If you are in the right hand lane you do not need to indicate until you are approaching the exit you are using ( if it may cause confusion you should not indicate). Indicating left to say you are going all the way around a roundabout is a courtesy to others but not necessary by law.
It is logical from their point of view but not how we are taught to treat roundabouts. Also it is not as efficient as the UK system as almost half of the roundabout is unused and everyone lines up behind the white car and waits.
Even knowing this it still winds me up sometimes and is one of the few things I do not like in Spain
To reiterate: If you approach a roundabout and treat it as you would one in the UK you will have near misses at best or a very unwelcome accident at worst which will be viewed as your fault by the police and the insurance so do not do this (the red car)
For more information I shall Look for the relevant pages on the DGT site