Because that should only be implicated. The oc_only tag is specifically for images where the only characters depicted are fan-made characters...you know, as opposed to images where there's a mix of fan-made characters and canon ones? Un-aliasing would allow users who don't mind oc's, as long as they're not hooking up with canon characters, to search specifically for oc_only and stop the rest of us hearing them bitching about it. It would also allow tag editors to more effectively help with removing the friendship_is_magic tag from images that, by rule, don't deserve it because they're oc-only images.

+1 but also I didnt know that was even a tag before now

(oc is aliased to fan_character) could be useful. Afaik there is no tag combo (mlp -[insert 500+ feckin names]) that would search for fan characters and exclude canon ones.