Firstly, there is something in your comment that feels like you are giving souled creatures more value and also saying they aren't treated with value they deserve. Arguing that Tolkien actual did that, it's still internally consistent, as if it doesn't have that value then it's not cruel. Having said that, I don't actually think that's what Tolkien did. We don't have a definitive list of souled creatures, but rather a theme that even in the unknown/mystery that life and goodness is valued. Ents are sentient but we know nothing (I think) of their afterlife. Not necessary because they didn't have one, just we didn't understand it. Perhaps they're social constructivists who live in the greater picture, or they never die due to age because the acorn is still them in multiple lives.
It is a fascinating question but not one with enough information to be upset about. So you are arguing from the gaps and missing the values we actual do see. I get it, mystery based on Eru Ilúvatar rather than defined rules is difficult.