"Unlike" in this case refers to the fact that you have "liked" this post, and now, due to a change of opinion, you decide to reverse that "like", by "un-liking" the post.
"Dislike" is not used here because "dislike" does not have the same effect that "unlike" has. "Unlike" is used in this case only to remove a "like", but "dislike" has the effect of not only removing the "like" but to actually have feelings of antipathy towards the post.
Putting it in SE style:
You see a post you believe is good, so you give a +1(like). After a while, you change opinions, so you remove your vote(unlike). Then, you think, this post is really bad, so you give it a -1(dislike)