I shan't comment on how realistic/viable a case of infringement is, admittedly personally I think it's unlikely but that is a layman's view!
What I do want to say... is that there is a
very important need to protect trademarks and intellectual property etc like in the NPHC Converse case where shoes (sneakers?) were sold marked with more than a passing resemblance to NPHC orgs founding years/colors, deliberately and without any permission.
I think that is very very different from what is the case here. I believe it is important to act to protect properties but in this case I think resources could be better focused than on this questionable area? I personally
FEEL, that to instigate a lawsuit in this situation would be symptomatic of the reputation the United States has (whether the reputation is poppycock or not!) for being sue happy over everything.
In reference to your last paragraph, I agree, but think
because it is so open to interpretation that suggests long drawn out - read expensive - lawsuits where the money could be better spent elsewhere.
I don't know how the fact the sources were attributed affects the whole issue if at all.