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.
Easy: Votes are an unlimited resource because a user can vote on as many posts as they want and a person can create basically unlimited user accounts, thus the fediverse would be like a market where everyone can create money out of thin air, defeating the purpose of having a market at all.
The fediverse would be more like a market if users had to “earn” votes by posting stuff other people vote on then “spend” those votes on other people’s posts. Then votes would be a limited resource that would make sense to apply market principles to.