Because by the end of the story Frodo has become a pacifist as well. But
Tolkien certainly wasn't arguing that pacifism is necessarily better
than fighting evil -- Gandalf and Aragorn are not pacifists, nor are Merry,
Pippin, and Sam at the end of the story. Actually, Gandalf might have
become as much a pacifist as Tom at the end of the story, when he relinquishes
his stewardship; as with Frodo, his job is done, and others can handle the
fighting from now on.
Tolkien seems to acknowledge the value of pacifism while also acknowledging the
value of fighting the good fight for a good cause. As a devout Christian
who fought in WWI, I would guess that Tolkien had personally struggled with the
question of whether there is ever a good reason to kill. And in his
letters he clearly condemns the notion that moral judgments can be abandoned as
soon as war is declared, and that orcish behavior on one side justifies equally
orcish behavior in response. He seems to respect pacifists who refuse to
fight more than he does those who are eager to fight, but fight like
orcs.
“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language. (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)
Tips for posting in the Reading Room.
