Yeah, the principled reason (though I am not like super confident of this) is that posts are almost always too big and have too many claims in them to make a single agree/disagree vote make sense. Inline reacts are the intended way for people to express agreement and disagreement on posts.
I am not super sure this is right, but I do want to avoid agreement/disagreement becoming disconnected from truth values, and I think applying them to elements that clearly don’t have a single truth value weakens that connection.
I explained it a bit here: https://www.lesswrong.com/posts/fjfWrKhEawwBGCTGs/a-simple-case-for-extreme-inner-misalignment?commentId=tXPrvXihTwp2hKYME