What is your opinion on deleting such a post?
I didn't read the post in its context and I haven't had a look at the thread, hence I'm not taking out the to comment on whether the one deleting it did a good job or not.
Is there anything to protect the user from being moderated unfairly?
A well-chosen set of moderators should do that job.
What if this moderator is a member, or has an alt account which is a member of the highest paying signature campaign which is for a site that is run unethically?
You're questioning the integrity of the moderators (or at least of one moderator, which you do not know specifically, hence we're back to all),
which I currently don't see a reason to. At least not for the sake of 1 deleted post.
Edit: This is in no way meant to come off as an attack on Lupin, but after re-reading it think it may come off that way.
Believe me when I say to you, I've had worse.