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Genealogy is by nature a nosy hobby. Like other academic subjects which involve the study of people, in order to participate, you have to stick your nose into other people's business.
Ethical researchers whose research involves living people try to achieve a balance between doing their work and preserving the privacy of the research subjects. One has to draw the line somewhere. It's common for researchers to remove personally identifying information when they publish their research.
The G&FH.SE guidelines follow the same underlying principles that govern the release of record collections (e.g in the USA, Social Security applications made by people that were born more recently than 100 years ago have some information redacted; census records in England and Wales are usually not released until 100 years after the census was taken).
Everyday etiquette also dictates that it is rude to talk about living people behind their backs. If you want to talk about the personal history of a living relative, go and talk to them. If they have a question about their own family, and to ask that question here would require them to relinquish their own privacy rights, the choice about what information to post should be theirs to make and not yours.
There is precedent for having restrictions for posting information about living individuals on the Internet.
Wikipedia's article: Wikipedia:Biographies of living persons, in the section Avoid misuse of primary sources, says, in part:
Do not use public records that include personal details, such as date
of birth, home value, traffic citations, vehicle registrations, and
home or business addresses.
In the section Privacy of personal information and using primary sources Wikipedia says:
With identity theft a serious ongoing concern, people increasingly
regard their full names and dates of birth as private.
Ancestry.com has restrictions on what can be posted on their online family trees:
In order to ensure the privacy of its members, Ancestry.com
automatically hides information for all living individuals. Only the
owner of the tree and the owner's selected guests can view living
individuals within a tree.
Note: Individuals are considered to be living if they are under 100
years old and have no information in the death date field.
Other sites online which have online trees undoubtedly have similar policies. The principle is the same: you can give away your own information, but it's not ethical to give away the right to privacy that belongs to someone else.