By Judy Vorfeld
In this case, we’re talking about attributive nouns, not compounded nouns. Merriam-Webster’s Collegiate Dictionary says an attributive noun acts as an adjective. It uses the word “city” in “city streets” as an example.
Chicago Manual of Style (15th Edition 7.27) says that the line between a possessive or genetive form and a noun used attributively (as an adjective) can be fuzzy, especially in the plural. It now dispenses with the apostrophe only in proper (often corporate) names, or where there’s clearly no possessive meaning.
Professor Charles Darling says, “One of the most difficult decisions to make about possessives and plurals of compound words occurs when you can’t decide whether the first noun in a compound structure is acting as a noun that ought to be showing possession or as what is called an attributive noun, essentially an adjective.”
Amy Einsohn, in The Copyeditor’s Handbook, says, “C15 7.27 discourages apostrophe-less attributive nouns (consumers’ group, not consumers group) but notes that the attributive is used in some names (Publishers Weekly, Diners Club).”