EU or not, you will be leaving Avis with a car with spanish registration plates in another country - Portugal. A lot of Portuguese will decline the car because unfortunately foreign cars attract thieves of and from the car. It also makes them a target on the road.

Unless Avis has a renter who is wanting to drop the car back in Spain, they are left with a car that is out of location and not earning. I would have thought that the same might apply to cars within the USA that were left in a different sate from the one in which it was rented. If such an AWD, I too would be pleased to learn of it.