I've had the exact opposite experience (i.e. getting downvotes for saying something in defense of the government). I think political threads tend to lean in one direction pretty quickly. I usually check HN after getting home from work, at which point most threads have already been going on for several hours. At that point, if a thread doesn't already have discussion from multiple sides by then, I don't bother commenting. I feel like at that point you're only going to get downvotes for disagreeing.
Yes. It's not the case every single time. I've been upvoted for negative comments about the government but I have to be much more indirect about it. If I'm blatant about it, I get downvoted to hell.