LotR -- Book 5 - Chapter 02 - The Passing of the Grey Company

[Nilpaurion Felagund]

I had this theory regarding the fate of the Dead Men:

It had been held lawful only for the King of Númenor to call Eru to witness, and then only on the most grave and solemn occasions. The line of the Kings came to an end in Ar-Pharazôn who perished in the Downfall; but Elendil Voronda was descended from Tar-Elendil the fourth King, and was held to be the rightful lord of the Faithful, who had taken no part in the rebellion of the Kings and had been preserved from destruction.

UT III 2 - endnote 44

Perhaps the people of the King of the Mountains had sworn an oath not dissimilar to the one sworn by Éorl to Cirion. And since both Isildur and Cirion called Eru in witness, any effects of those oaths will be upheld by Eru. (Me--from Strange Curses!)

Of course, why Eru would intervene on behalf of Isildur is something I do not know. Unless he knows that someday, some ragged, rugged King-wannabe would want ride off to his rightful kingdom with some sort of an army. I think this goes in with his style, for if you remember he tells Melkor (and perhaps that word is for all who follow in his path) that:

. . . [T]hou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined

Ainulindalë