Okay, well I have lived this one pretty recently so let me give it a shot.
The first stage of grief is DENIAL. It is easy to be happy as can be when you are in denial about a loss you have suffered. When I came to this site I was in total denial.
The second stage of grief is ANGER. Ok, so this is when you get pissed because you are starting to acknowledge that some of this "stuff" makes sense, some of it resonates. You lash out at the truth because you don't want to accept it. I suspect that some of those you speak of are at this point.
I'm not trying to accuse any adoptee of being in a fog or being in denial, I'm just saying I WAS. Had I been as secure as I pretended to be I would have visited this site and left after one time thinking everyone was crazy. In fact, for most of life I was secure and gave all this little thought. You couldn't get me mad about adoption because I simply didn't care.
I think if someone is here they do care. If they are mad, something is making them uncomfortable. They are investing the time here for a reason.
I do think there are adoptees who are fine with their adoption, who don't struggle. They may be this way for a portion of their lives or forever. AND my observation is they aren't happy, sad, or angry- they are ambivalent and they certainly aren't hanging out on a website arguing with strangers. I would ask those angry-happy adoptees to really ask themselves why they are here? There is nothing wrong with being angry, just don't claim to be so dang happy while you're at it.