I think the Iceland's in Spain are franchises. I presume they use the same distribution channels as the UK Stores, where the food is brought in from the warehouse in the UK. So, they have little choice because they are reliant upon whatever Iceland in the UK buys.
If you want to buy Spanish produce in Spain, don't go to Iceland!

Go to a Spanish supermarket, such as Mercadona. Even better, they have great local markets even in so called tourist places.
Iceland are very much catering for British people in Spain. I realise there are other 'foreign' supermarkets in Spain such as Lidl and Aldi (German), and Carrefour (French) but they have many more stores and are registered in Spain, thus selling more local produce, as they are catering more for the Spanish market (rather than ex-pats or holidaymakers).