Why can catapults damage players but not NPCs? In my opinion, if you are a small-time solo player and if you are willing to invest the resources, I see no problem with being able to catapult a mammoth, a troll, or a whale to death. It would take many shots, and it is not like these creatures are gamebreaking should the average player get their hands on one. Hardcapped by the shooter's survival.
Any competant group grinds these mobs on a near daily basis like it's nothing, so why not give more options to people who are not combat-oriented or who play alone?
An argument I can see against this would be that people could potentially catapult livestock from outside of someones' walls, but placing your animals 25 tiles in doesn't seem like much to ask and even if your animals were in range, only 3 shots can be fired at a time from 1 catapult and it is unlikely for multiple catapults to be used against random hermits. There is also the option to simply not allow tamed animals to be catapulted.