I reviewed your report history, and out of thirteen images you’ve reported for Rule #5, it looks like eleven of them have been deleted for Rule #5. I certainly appreciate you reporting such images, and it appears that your reports are pretty accurate most of the time. However, that doesn’t mean that we’re gonna agree with your assessment 100% of the time, nor does it mean that the site’s moderators are doing whatever they want.
If an image is tagged both “anthro” and “foalcon” by the artist, either on this site or at the source, it’s an instant deletion - no questions asked, no further evaluation. If the artist says it’s anthro foalcon, that trumps everything else and the image is deleted from Derpibooru. That’s why Anibaru’s picture was removed - he tagged it as such at the source, so it’s not allowed here.
As for the Diamond Tiara image in question - I’m not in a place where I can be reviewing NSFW images at the moment, but we’re generally on the “better safe than sorry” side of things when it comes to Rule #5, and typically tend to delete more than we allow, so if multiple moderators are saying it’s fine, it’s probably ok. I’ll review it later when I have a better chance to.