The fact that it is free software shouldn't make things different since it's a matter of copyright law. It's fine to copy the interface - the implementation not really. There would certainly be NO problem if a close reimplementation was under the GNU GPL (as a derivive work). But a close reimplementation can't have a different license.
This was a discussion of the philosophical/meta/attitudinal issues, and why this is kinda a bizarre attitude for a free software project to hold.
Legally -- implicit in your comment are unstated assumptions about what is copied from the implementation. Suffice to say -- I disagree that a so-called "clean room" implementation is always required re: software published on the internet, or that the act of simply reading GPL code taints one forever, re: that code. The reason why is the reason why reading a book does not taint one forever and prevent one from writing one's own book -- "You should have used a while loop instead of a for loop here, now you're going to copyright prison." See, and pay close attention to the "Merger Doctrine": https://en.wikipedia.org/wiki/Idea–expression_distinction
It's my view that copyright is a rather weak IP protection, and GPL-type advocates have misconstrued the law of copyright for years, especially "derivative works". Mostly because they've been misinformed about the law by mendacious FOSS leadership cough like the FSF and SFC.
Now I'd agree with you, if there is something especially clever/creative about the GNU implementation that the reimplementation copies, that might be an issue, but again, even then, it seems especially rich for a reimplementation to find fault with another reimplementation.
a reimplementation that copies the interface has no need to copy the license. GPL people just get scared because they know their license can't prevent people from rewriting their own software (of course, such an idea is ridiculous).
the GPL is not a perfect license, there is no such thing.