"Service counter" is a compound noun; it should traditionally be "service-counter," with a hyphen.
It isn't "counter-service" because the word-order would imply that "counting" were provided as a service.
The idea that the first word in such a construction be an adjective is an increasingly common misconception. Compound nouns are a common occurrence in about all Germanic languages; they are two nouns that, together, form a more specific meaning.
It's an easy and empirically-forgivable (compound adjective!) mistake, but a mistake nonetheless.