The verb halten in combination with für forms a kind of "Phrasal verb", in other words the combination takes on a different meaning than the meaning of the two words together would imply. (With a separable verb, in contrast, the preposition does not have to be part of a prepositional phase. I don't know the official name for these things, but "Phrasal verb" seems descriptive.) There are are a number of them, for example einladen with auf -- "treat to", richten with auf -- "point at", sehen with nach -- "see about". In this case halten with für means "take for", as in believing something is something else. For example Hält er mich für einen Trottel? -- "Does he take me for a fool?" (from Wiktionary). You can also put an adjective in place of the something else to get something like your example: Ich halte es für richtig ... -- "I take it as right ..." So it's the unusual circumstance of this one verb plus preposition combination which happens to allow a adjective to take the place of one of the nouns that make the für + adjective possible here; it doesn't normally occur.