As stipulated in the quote you quoted, because their false joy ought to not matter to us that battle with them. Even a little bit. Especially if we are somehow seen as the source of that false joy.
To me, it'd be like them saying, 'it would bring us joy if you elected Hillary Clinton as your leader.' And then each time someone says, "Vote for Hillary!" I could say but this is bringing joy to our enemy. And you'd say, no no no, don't vote for Hillary, because our enemy said this would bring them joy. And we don't wanna do that!