My knowledge in Euskara is hardly basic, but as I have some appetite for linguistics, I was interested in your unanswered question and I wished to look for some data on the point. There are some cases where adjectives don't take a determiner even though they refer to a definite noun : enumeration of adjectives, left out auxiliaries, phrases with a general meaning . Some adjectives have a different meaning if they are determined or undetermined as bizi da (he is alive) / bizia da (he is lively). But your examples don't seem to belong to those cases.
Looking for some examples in on-line dictionaries, I have come across some surprising ones as well : Zoroniez, komunak libro zeuden ( Fortunately the toilets were free ) ; une honetan ez da libre ( He is not free at the moment ) , but idazlanak gai libroa da ( The subject of the essay is free) ; hegazkina bi hilabete barru prest izango da ( The plane will be ready in two months) ; bizia kario da Parisen ( The living is expensive in Paris ) , but holako bizia karioa da (Such a living is expensive) . Is it due to the fact that these adjectives are loan words from Spanish or French and to regional variants ? Maybe. I've also read the following translations in the same dictionary : The sky is blue => zerua urdin(a) or zerua urdin dago. The defined form could mean the sky is naturally blue and the undefined it is blue right now , the same difference expressed in Spanish by the auxiliaries ser and estar. I am just hypothesizing !
Hope a native clears up the question.
