The ability to lock your posts to a subset of your readers creates an expectation, and culture, of privacy. The ability to broadcast comments made in locked posts breaks that expectation and defies the norms of that culture.
Telling people "you shouldn't put anything anywhere on the internet you don't want someone else to read" is blaming the victim. The weaker version, "if you don't trust people not to abuse this then don't friend them," is still blaming the victim. Don't do it.
Telling people "you shouldn't put anything anywhere on the internet you don't want someone else to read" is blaming the victim. The weaker version, "if you don't trust people not to abuse this then don't friend them," is still blaming the victim. Don't do it.