I don't disagree, but that's a normative argument about what addons we should choose to use.
My concern, however, is Mozilla putting themselves in the position of making that judgement for their users. Even if we agree that this isn't a good addon, does that mean it's fine for it to be censored by Mozilla?
The guidelines outright state that "Any add-ons, or add-on content, hosted on Mozilla site(s) must conform to the laws of the United States." The issue is that this addon falls in the murky category of neither being clearly legal nor clearly illegal. I suspect that the addon reviewer (who most likely does not work for Mozilla and is only a community volunteer!) felt that an addon advertising its purpose to get around copyright controls would have pushed it over the edge to illegal, even if it's only taking actions that are themselves legal.
My concern, however, is Mozilla putting themselves in the position of making that judgement for their users. Even if we agree that this isn't a good addon, does that mean it's fine for it to be censored by Mozilla?