AFAIK Jake Appelbaum is a US citizen.
They can't actually refuse entry to citizens, but they sure as hell can interrogate/threaten/harass/etc. I know Jake's been harassed a bunch of times at the border (and had his laptops/phones/etc stolen under the guise of searching them - they get to keep them for 48 hours to forensically image but then never give them back).
It's happened to me, too, and I'm not even on any of these lists, I just choose not to answer any of the voluntary questions they ask of everyone coming in. Anyone who values their freedom and has watched the excellent Don't Talk To The Police video will do the same, and will meet with similar harassment and abuse.
Cops in the USA are real dicks, especially the super-"patriotic" border cops, and they can and will make your day really terrible if you don't do exactly as they say (even if you have the legal right not to).