There is a "rule" which permits both singular and plural attributives (pre-head internal dependents). On pages 439 and 444 in The Cambridge Grammar of the English Language by Huddleston and Pullum (2002), it says, respectively: Pre-head complements are usually realized by nominals and Internal modifiers in pre-head position are realised by [...] nominals in plain and genitive case. Neither of the "rules" places any restrictions on inflection regarding their number. Here's a little bit on the difference between modifiers and complements.
Regarding everything else – it's up to what sounds better to English-speaking people. A rule of thumb could probably be: if it normally comes not as a single item, but as a multitude of items/phenomena/whatever, then that's how it could be denoted in writing (i.e., in the plural).
(transcribed from comments)