Wow, that's a lot of (possibly rhetorical) questions to answer. Okay:
1.) By that phrase "the anarchist community," I don't mean to imply that there is or ever has been a single big happy family constituted by all the people who call themselves "anarchists" -- but communities are not ordinarily single big happy families; they are groupings constituted by people who have something in common, e.g., problems, interests, desires, ideas, and who feel some sense of mutual solidarity as a result. Communities are always fuzzily defined, so that it is often difficult to tell where they begin and end, and they are always full of differences. These are some of my basic assumptions, anyway. On that kind of understanding, I think we can probably agree that something we could call an "anarchist community" is constituted by people who have in common at least a few things, e.g, we call ourselves by the A-word _and_ we enact that in some concrete ways (associate with other anarchists, create/participate in anarchist actions or projects or organizations or institutions, read and write anarchist texts, try to live in an anarchist way, etc., etc.). None of this fulfills the conditions implied in your question, "what group is that exactly?" -- but communities aren't generally capable of being "exactly," period.
2.) The event at which said pieing occurred was the SF Anarchist Bookfair, yes? As opposed to the I-Don't-Know,-Some-Kind-of-Politics Bookfair? Okay then: I'd say that what happens at such an event, all things being equal, can be taken to reflect in some way on the community by and for which it is created. For example (a much more dramatic and awful example), I think of Altamont: the beating murder that happened there was not the work of the festival's organizers, nor was it perpetrated by people who were typical of the hippie community by and for which the event was created, but the fact that it could happen there and that nobody stopped it sent a message, cast a pall that spread way past that particular time and place. For a lot of young people, it signaled the fraying of that community and the dispersal of the hopes and energies that had gone into it. In the case of the SF Anarchist Bookfair, even if "non-anarchists" were the only ones directly involved in the pieing, it is hard to disconnect it from the anarchist community because of the overall context, because of how the organizers handled it, and because there was cheering -- and no other response from the participants.
3.) It's been observed that people are dependent on authorities and oppressive institutions to the degree that they are unable to depend on each other. Where trust and solidarity is absent, force and coercion fill the gap. Sadly, this seems to me to describe the situation at the SF Anarchist Bookfair. Even if Lierre Keith is not an anarchist herself, even if (I do not know, not having read it) her book is offensive to some, the positions she was expressing when she was pied were not atypical in the anarchist community -- no? I have only my own experience to go on, but if every anarchist who is opposed to factory farming but not to eating meat were to be the recipient of a pie . . . there would be a lot more pies flying, don't you think? Note that while pie-throwing isn't necessarily violent in the narrow sense (though it's quite possible to hurt people in the act), it is a gesture of supreme contempt; it signals that the target is worthy of maximum disrespect. In a community in which some large fraction (half? a third? two thirds? what do you estimate?) have reason to believe that they are going to receive maximum disrespect from the other fraction (a majority? a substantial minority?), how much solidarity, how much cohesiveness, how much energy-to-do-work-together, in short, how much _power_ is it capable of generating?
Please, tell me if I'm missing some part of the picture, or if you think I'm mistaken in my interpretation, or if this doesn't fit with your experience. I'm being sincere, though. I don't think this is an isolated incident; I think it's a symptom of bigger problems we have, ethical problems, problems of negotiating across differences. We risk shrinking our community, already small, into even smaller communities of even narrower consensus. It's not all a matter of narrowness, either: we have good reason to be afraid of erring on the other side, the side of tolerating too much in the name of community or solidarity, looking the other way when folks who share the same political identification pull sexist or racist or otherwise oppressive crap. But I know many sincere, engaged anarchists who are routinely afraid of being judged and disrespected and attacked by other anarchists for what anybody outside of the community would consider to be really small differences. I don't think that's a sign of health. Do you?