I see no requirement of any kind for any word or action at all to be necessary to show tacit approval for something. In fact the common thread is silence. Most often tacit approval is indicated by a failure to deny or disapprove when the opportunity to do so is presented.
Argument from definition, what fun!
1.understood without being openly expressed; implied: tacit approval -- you think you can glean understanding from silence?
2.silent; saying nothing: a tacit partner -- You think a tacit partner takes no actions to become a partner?
3.unvoiced or unspoken: a tacit prayer -- you think every instance of silence is a prayer in this example?
1: expressed or carried on without words or speech <the blush was a tacit answer — Bram Stoker> -- a blush is a visible indication, but not a verbal one
2: implied or indicated (as by an act or by silence) but not actually expressed <tacit consent> <tacit admission of guilt> -- again, you think mere silence can imply something?
Now, if I were in teh regular habit of saying posters A, B, and C were not trolls, and declined to make such a comment for Millsapa, that I would agree that silence has an implication by its contrast to other statements. However, I don't regularly make such statements.
Here's a good comparison:
Millsapa: Yes, the anti-abortion crowd is just out to ruin your ability to have irresponsible casual sex with teenagers and young women.
One Brow: Yup.
Note I am silent on any personal desire to have irresponsible casual sex with teenagers and young women. Can you imply anytihing about my desire from that silence?