That's right, it has always bothered me that certain animals are killed en masse without a good justification. The case you explain in this publication is one of thousands that occur in the world, manatees are also another group of marine animals that are in danger of extinction for various reasons such as hunting, water pollution or habitat loss, there are even places where they can hardly be seen.
On the other hand, it is said that rhinos are not affected in their health by the removal of their horns, but some experts suggest that removing the horn to these animals can affect their behavior, as each part of a living being has a function, in rhinos the horn can serve as a defense against predators, protect territories, and can even be used for mate selection, the latter being the one that may be compromised. Although I don't know much about these animals, since my area is insects or arthropods hahaha, but I understand that the horn will grow back over time, so that measure to prevent hunting is temporary and aims to further extend the life of the rhino. According to what I read at the time, a kg of horn is worth $17,000 in Asia, but I don't know how true that is. As you say, it is unfortunate that these measures have to be taken to protect these animals of our own species.