This question is an exact duplicate of:
Where can I learn about the English rules for noun phrases in which a modifying noun is semantically plural? We say "Horse trainer," not "horses trainer", even though there is usually more than one horse.
In some rare cases, however, the plural noun is used. I vaguely recall Steven Pinker saying something about such cases -- perhaps it involves phrases where the plural noun is irregular or takes on its own lexemic status.
I'd appreciate a summary of this and a pointer to an article.