ask any old-timer fanfiction writer about this. "fan work" as a whole--including mods--is a gigantic gray space in current copyright law and IP holders are almost certainly within their technical legal rights to prohibit any works like Revolution on a blanket basis. the current arrangement where most rightsholders look the other way and/or accept the existence of such fan works is a largely informal one, and there's nothing codifying it being that way. it could arbitrarily change (or just be fucked up by a court case) at pretty much any time--and, indeed, occasionally rightsholders still do try and enforce their IP quite aggressively.