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.
You can’t force people to be positive, they might still leave negative comments.
Lemmy is still small, so users don’t have the option to choose servers as they please. For instance, I’m looking for an instance where you don’t need an email to register (for privacy reasons) and where you can quickly create an account (I don’t have the patience to wait for days to get my account). Also, it should have a good number of federated servers working with it.
With these specific requirements, there are only a few servers available.