Lets just say User A does something -> It appears in User B's feed.
If you remove the item from A's feed when B deletes it, A will wonder what happened to the item that was just in their feed. If A has a lot of "friends" or whatever, they will often find items disappearing. As A's number of "friends" approaches infinity, their feed will become completely empty. This doesn't make sense.
Conversely, it does make sense to remove the item from B's feed when A deletes it. We can presume that A had a good reason to hide their action from the feed and it would be good to hide it from everyone else.
I guess that it makes sense for only the owner of an item to be able to hide it from other people's feeds.
Back around to the phototagging example, you might decide that it would be a good idea for the subject of a phototag to be its owner, since it identifies them in what might be a compromising situation. In this case, since B is the owner of the phototag even though they didn't create it, B deleting it removes it from everybody's feeds.
Above, B deleting the phototag removes it from everybody's feeds. Nobody knows what happened at the office christmas party. However, since A created the tag in the first place, we can assume that they do know, and hiding it from A is useless.
tl;dr
Basic principle: Only the owner of an item can delete it from other's feeds.
Special case: The subject of a phototag, not its creator, is considered to be its owner.
Special special case: Hiding a phototag from it's creator will not destroy their memory of having created it, so hiding a phototag from its creator is useless.
