That, and the decision is one-sided. Say I want to post something publicly about some new technology. Do I post it publicly and have it go into my family's feeds where it will be considered akin to spam? What if one of my friends actually _is_ interested in new tech but I don't know it?
I get that it would be an added layer of indirection, but to allow each user to have multiple subject personas for posting and let others subscribe to said personas might have been more useful. As it stands, I err on the side of caution and post privately to the people I can best guess might be interested.