UPDATE: Newer post about this here.
This is very preliminary information and I only had the most tertiary review of the subject but one of my peers said that an email from Zazzle indicated they were flagging, to some degree, designs that might be considered stronger than PG-13. The preference being toward more family-friendly PG-13 or milder designs.
We’ll see what comes of all of this. The Zazzle forums will surely be abuzz talking about it sooner or later.