I absolutely agree with you. In fact, just 1 or 2 hours ago, I sent a mail to the site admins about this.

I had also previously sent a mail to the site admins about one newsletter that came out on October 3rd, which also had a really horrible skull with worms all over, or whatever it was. Again, no warning, nothing, just bang! on my face. Disgusting.
I also receive newsletters from other similar sites (contentparadise, 3dcommune, runtimedna, etc) and Renderosity was clearly the most offensive.

Like I wrote in my mail to the admins, this site has a multi-cultural audience and Halloween is a mono-cultural tradition. I find it completely wrong to force it in such a gross way to everyone.