I believe this is an incorrect description of trademark law.
I'm not an attorney, but my understanding of trademark law is that it protects against uses likely to generate confusion among members of the general public. It's unlikely that anyone seeing a brothel with the same logo as Meta-Wiki is going to assume that the two entities legally affiliated.
You can have an "Apple Computer", "Apple Records" and "Apple Dry Cleaning". Each of them can have an apple image in their logo. However, they can't each have The Same apple image in their logo because the specific logo still identifies a specific company.
All of those logos (well, the first two, at least) are copyrighted. The image in question is in the public domain.
In other words, using the exact logo is by definition implying an association, just as if they had used the word "Wikimedia" on their product.
Companies called "Peace Computer", "Peace Records" and "Peace Dry Cleaning" can all use exactly the same public-domain CND peace symbol as their logos without committing trademark infringement.
And of course, the image in question has been in the public domain since 2006, so there might be many uses that predate Meta-Wiki's.
But as I noted, I'm not an attorney. My only concern is that Foundation policy is being met. If it is (and the Foundation is okay with Meta-Wiki having a PD logo), that's good enough for me.