The most prevalent is probably group mentality, a majority are "against" fanfiction so the majority will continue that in case they are branded as an outcast. It's cyclical and probably the core of it being an "Acceptable Target" as a biting comment against a fanfic writer is unlikely to be contested in most anime communities.

Another major problem is the barrier for entry for fanfiction is so low compared to other fandom you mention like cosplay and AMV. It takes very little skill to write badly, but it takes a greater amount of skill to be bad at cosplay and AMVs (if that makes sense). So while I could write terrible fanfiction in this comment box, I would need a sewing machine, material pattern for cosplay, or Adobe Premiere and timing skills for AMVs.

Another significant area could be the perceived lack of creativity some attribute to fanfiction. As if having an established world, set of characters and canon somehow gives a "free pass" to writing fanfiction. This is of course not the case, but is difficult to convey to non-writers without concrete examples. AMVs and Cosplay are visual by nature meaning their effect is immediate rather than slow burning.

There are probably plenty of other reasons for what you describe and unfortunately, no get-out clause in sight.

This strikes me as singularly unfair.

If you mean the quality of the fanfic, I did mention Sturgeon's Law. It applies to a great many other fields, and I don't really see why fanfiction (among other arbitrary groups) gets such a bad reputation, and other (also arbitrary) groups do not.

It could well be an Acceptable Target, but I don't know why it's an Acceptable Target.

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