I don't have anything close to an explanation as to why some people received the newsletter(s) and others did not, presuming equal subscription rates. I do seriously doubt that game or forum activity has anything to do with it.
Here's my own experience, so far:
I have two "main" accounts, the one I first created back in April 2009 (this one) and another which was created in 2011. The first account is/was signed up for the newsletter for both US and EU, the second account for US newsletter only.
I received an EU newsletter on the first account, which was suspiciously blank. No text or graphics at all, no Pingou or Zodiac code, absolutely midnight black background. The newsletter was from "Gameforge" and entitled "Runes of Magic EU". I'm not sure why I didn't also receive a US newsletter on that account, but it's possible that it was canceled out by the EU newsletter. Go figure.
I received a US newsletter on the second account, with graphics and Pingou code intact. The newsletter was from "Gameforge" and entitled "Runes of Magic US". Again, go figure.
I have other accounts which I have not checked yet, but I'm going to guess that since they were all subscribed to receive newsletters, the codes will be there.
I've not yet redeemed the Pingou code that I received, and it doesn't matter to me that it's for a physical DPS transformation potion. The pet for the character where it would be used is a DPS pet, the character is a P/S/R. I'm not sure I want a Linux representative following me around.
Just my 1.34 cents--the rest of it falls under Sequestration. The current Administration has taken it to pay for White House tours that have been canceled.