Hello Kangiten

I'm a learner from Japan. Your question is exactly what I too have long thought over but I have not still got a decisive answer for it. But up to now I have a feeling that there is no definite rule in forming noun-noun compound noun phrases. It is true that in the case of "right" the noun is usually expressed as "rights" when it is used as an adjective (e.g., "human rights protection", "human rights movement"). But, I feel it is rather an exceptional case. In most cases, nouns are expressed in singular forms when they are used as adjectives. Let me take "a coin collector" as an example. "A coin collector" means a person who is collecting coins. A person who has only one single coin can't be a coin collector. But still we say "a coin collector" rather than "a coins collector". Likewise we say "a file binder" rather than "a files binder". Let's take another example. "A parent meeting is held once every semester." In this case, logically more than a single parent should come to the meeting, and so, in fact, some people say "a parents meeting", and some people say "a parents' meeting". But still "parent meeting" is the commonest phrase. Similarly "a student union" is commoner than "a students union" or "a students' union". So my rule of thumb is "use a singular form when the noun is used as an adjective".

Of course, there are many exceptions. For example, "United Nations" or "United States" are always in plural even when it stands as an adjective. It may be because those phrases would loose their own sense if they are used in singular forms. As for the case of "rights management", I think the reason this form is preferred could be that if we say "right management" instead of "rights management", one might take the meaning as "the management is right". But it's my humble opinion, and I might be wrong as usual.