By default, Lemmy allows downvotes globally. However, when a server disables downvoting, it is similar to using a feature that is usually reserved for enterprises and very small, non-federated communities.
If a user prefer to not see downvotes, they can disable it by his favourite client settings, but the rest of the community should not miss this functionality for the pleasure of few users.
It would make sense if the end result was to prevent downvotes on NSFW posts by any user, but that’s not how it works in practice. My lemmynsfw account can’t downvote anything, but my other accounts can downvote anything (federated).