Well, "for" has many meanings. For sure "dla" would be used when it's "for someone" (A gift for you), "za" is for example used with paying 'for something' (płacę za samochód)... generally, I believe you have to remember different usages and contexts.
We actually accept it, it should have worked. But we're really not fond of answers ending with a pronoun. Still, given that it's a whole prepositional phrase ("dla nas"), it's a bit better than usually.