The point of the protest is not only to let your regulars know (as you pointed out, they already know) but to let the world knows.
How?
When all the tech sites go dark, one can say: look, the entire tech community is strongly opposed to this thing, just look at these sites <insert list of links to high profile technology websites>.
Blacking out HN would be like switching off your computer in the privacy of your own home. Nobody knows or cares. All that would achieve is to remove an important vector for communication and information spread in the tech community.
The UK have been trying to pass a load of crazy crap like this. If the US pass it then we will follow.
This is why everyone should be protesting against regardless of your geographical location, unless you want a web in which every country sees different.