[netbsdusers]

There's no legal concept called "tainting". Black boxing is just a means by which you try to make yourself completely irreproachable (for if you haven't seen something, then it's outright impossible that you copied it). It obviously doesn't follow that the converse is true. If it were, musicians would not listen to other's music nor would painters look at other's paintings!

[schoen]

The part that they're trying to avoid is the "access" in "access and substantial similarity"

https://en.wikipedia.org/wiki/Substantial_similarity

(usual factual elements in determining the possibility of a copyright infringement in U.S. law).

I agree with you that it's possible in principle that copyright infringement would not be found even when there was evidence of access. But I think the courts would usually give the defendant a higher burden in that case. You can see in the Wikipedia article that there has been debate about whether access becomes more relevant when the similarity is greater and less relevant when the similarity is less (apparently the current Ninth Circuit standard on that is "no"?).