Maybe:
* This account already scored poorly on spamminess and attempting to post a bare link (with no content) on their page pushed them over the edge?
* The EFF Privacy Tips link was somehow used in a parallel spam campaign, or some characteristic of the page itself causes it to be flagged as spammy?
* Something about this specific browser session was flagged, preventing posting? (this might go along with the No Violations observation about the page).
I find it much more interesting to try to reverse engineer these strange behaviors of some spam detection systems than to just chalk them up to a "Facebook hates the EFF" conspiracy theory - if this happened to me, I'd be doing some more tests to try to see what's up.