Because the film is — actually, the original is unrated/XXX. However, an R-rated cut does not exist officially for this title. What likely happened is confusion with a regular R-rated pirate film or a fan edit. If you truly want a "guide" for the R-rated version of Pirates 2 , it would have to be hypothetical or an edited version removing explicit content.
It looks like you're referencing a specific, obscure title: . Pirates 2 Stagnettis Revenge 2008 STV Rated R V...
(sword fights, stunts, explosions) — fine for R rating. Because the film is — actually, the original