Given Google's emphasis on quantifying every aspect of recruitment, I wouldn't be surprised if the reason is because their numbers show that contacting people who previously rejected offers was more likely to result in a hire than contacting people who never interviewed, and that they decided that ignoring "do not contact" instructions was worth it after some time. Basically, kind of an retargeting strategy but applied to hiring. I understand why this may be annoying to you, but your case is probably rare enough that they decided to go along with it -- I would think that most people do not care enough about being contacted by companies they do not want to work for to complain. I do not condone this but the assumption that you may not feel as adamant about not working at Google as you did two years ago (during which a lot can happen) seems perfectly understable to me.
With that said, your post does come across as really obnoxious. Asking to have an "ON PAIN OF DEATH" mention added on their file? Really? I dislike having my instructions disobeyed as much as the next guy, but if I counted properly this is 6 emails over more than 7 years. Surely one could live with receiving one unwanted email every year or so? It's not like it's that hard or time consuming to just ignore a message or to decline politely -- after all, a lot of people do it multiple times on a daily basis, and I'm sure you do too.
This is too bad, because the tone of your post distracts from the perfectly valid (but IMHO, minor) point that this policy does imply things about Google they view themselves.