Copyright is limited in time (at least in theory!).

U.S. government works are not eligible for copyright.

Some works do not have sufficient creative element to be covered by copyright, or the other user may be using only the non-covered elements of the work (factual collections, or see recipes).

But probably most important, fair use is a limitation to the exclusive rights of copyright, in spite of copyright holders' frequent assertions that "any other use without written consent is prohibited". A use may be fair even against the express wishes of a valid copyright holder, as fair use is not infringement. Factors to be evaluated in fair use include, but are limited to:

  (1)the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;

  (2)the nature of the copyrighted work;

  (3)the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and

  (4)the effect of the use upon the potential market for or value of the copyrighted work.

https://www.copyright.gov/fair-use/

https://www.law.cornell.edu/uscode/text/17/107 see 107-112 for other limitations of copyright.

That's the catch though isn't it? It's only wilful infringement if there was intent. Just because you don't know the law, that doesn't make it allowed but it does make it unintentional.

A license notice provides a clear explanation of the rights and responsibilities of the user. This dissuades most any notion of "lack of intent" as the restrictions are put in your face from the start.

Hypothetical situation:

I have a bunch of code in a repository or set of repositories. I got confused and thought this file was one we created/had a license to reuse rather than a piece of third-party code, and thus copied it into a different project.