I have had a number of products with public domain images removed. All are images taken from trusted sites that state that the images are no longer in copyright are now public domain and freely available to use commercially.
The same images are also widely available on Zazzle competitor sites and stock photography sites.
Who else is unhappy with this aspect of Zazzle and their time being wasted 
Without knowing the images, it might be something in the title, description, or tags that falls foul of the rules, rather than the image.
“Commercial” doesn’t always mean that you can make a direct profit from it. Such as selling it on something POD. One example, “commercial” can also mean a for profit company can use the image in a newsletter they offer for free, but the newsletter talks about a service they do sell. There are several different types of “for commercial use”.
The solution is not to bother uploading to Zazzle, there are other pod sites with the same image so I will investigate loading images there.
If you are sure that the images are public domain, then it probably is a tag or a word in your title. Can you ask Zazzle for more clarification?
Yes that is a possibility, I say this as both here and Redbubble photo I have taken in Patagonia get flagged, no doubt due to the clothing company.