1) Muggings - If you get mugged by someone you know, it's very easy to find them and punish them. It's no coincidence that muggings tend to happen in big cities where the chances of a mugger attacking someone they know is very low (anonymity).
2) Sexual predator - If a priest rapes a child and the child tells people about it (i.e. removing some of the priest's anonymity regarding his actions) the priest is likely to be investigated and thrown in jail. If you know a sexual predator lives next to the school your kid goes to (i.e. he has lost some of his anonymity) you might take additional protective measures that could prevent your children from being assaulted. Whether this is the most common scenario, or whether it is fair for sexual predators to be publicly identified, are completely separate issues. I am simply illustrating a cost.
3) Terrorists - If you were the pilot of a plane and knew that some of your passengers were members of an extremist group that had plotted to destroy airlines in the past (i.e. some of the passengers had lost some of their anonymity) you would probably kick them off your plane. Whether or not this relates to 9/11 is not the point, this is simply an example of a cost of anonymity.
As for your last point: I never said the costs of anonymity outweigh the benefits, I simply showed that there are costs.
Intriguingly, without anonymity there is no real reason to prevent predators from living next to elementary schools nor interacting publicly with children.
Everyone around them would know their history, and name then therefore watch them to a close degree. As it is now, we simply try to prevent convicted pedophiles from being near unsuspecting children and parents.
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With a terrorist, the argument is the same. If one knows that party A is a known terrorist, and has thoroughly checked them out then why not allow them to board a plan. 100% security isn't demanded---if it were, then we'd simply lock up every known terrorists before they committed any additional overt acts, then never release them.
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Of all your cases, the mugging is the only one that makes sense because it refers to a lack of anonymity deterring a potential criminal rather than it being used as advanced warning for the potential victim.
I submit that it is not. How would your non-anonymous world stop a madman like Anders Breivik? He did not hide his views. He was a known user of white supremacist sites, and a known affiliate of white supremacist groups. However, no one, not even the fringe groups that he affiliated with could predict that he would carry out his horrible actions.
Breivik was not anonymous. That fact, however, did nothing to stop him from carrying out his crime.
I am not claiming that anonymity is the source of all crime.
Perhaps some of the misunderstanding is due to the broad definition of anonymity I'm using. Anonymity as I use it is the inability to link a person's actions to their identity. This type of anonymity breaks a lot of our social institutions. For example, if you don't know that a person committed a crime it's hard to arrest them.
If Anders Breivik was anonymous he would still be wandering Norway because no one would know that he was responsible for the murders. Some of the information I've read suggests that he wanted (or at least expected) to be caught. In such situations anonymity may actually discourage crime. I would guess, however, that most criminals would rather not be caught.
The argument I'm responding to states that if you know someone is a terrorist, then you simply prevent them from boarding planes.
That anonymity is the source of crime isn't my argument, it's his.
Clearly, you can be identified as an 'enemy' and still be dangerous. A good example is during time of war, soldiers are often uniformed and standing in ranks. There is no doubt about which 'side' someone facing you is on, and yet they are no less dangerous because of it.
Anonymity doesn't remove danger, but merely identifies that danger exists.
Wouldn't it be interesting if you could flag something to notify you if you're walking by a white supremacist?
1) Muggings - If you get mugged by someone you know, it's very easy to find them and punish them. It's no coincidence that muggings tend to happen in big cities where the chances of a mugger attacking someone they know is very low (anonymity).
2) Sexual predator - If a priest rapes a child and the child tells people about it (i.e. removing some of the priest's anonymity regarding his actions) the priest is likely to be investigated and thrown in jail. If you know a sexual predator lives next to the school your kid goes to (i.e. he has lost some of his anonymity) you might take additional protective measures that could prevent your children from being assaulted. Whether this is the most common scenario, or whether it is fair for sexual predators to be publicly identified, are completely separate issues. I am simply illustrating a cost.
3) Terrorists - If you were the pilot of a plane and knew that some of your passengers were members of an extremist group that had plotted to destroy airlines in the past (i.e. some of the passengers had lost some of their anonymity) you would probably kick them off your plane. Whether or not this relates to 9/11 is not the point, this is simply an example of a cost of anonymity.
As for your last point: I never said the costs of anonymity outweigh the benefits, I simply showed that there are costs.