Students plagiarising assignments: What level is worth the effort of pursuing?
I'd like to base my answer on the following piece of the question:


  […] but there seems to be a general reluctance in academia to do anything [about plagiarism], even in extreme cases.


This does not seem to be true. In cases of plagiarism in journal publications, theses or otherwise published material  academia is quite strict and careers have ended because of plagiarism (not only academic careers - at least two ministers in Germany had to resign from office due to plagiarism in the PhD theses which have been written years, even decades, in the past).

Things are indeed different when it comes to homework assignments and there are various reasons. Homework assignments do often repeat and the same assignments may have been assigned in the same form a lot of times in different places. So it is actually likely that different people come up the similar or almost similar solutions. Also, it is hard to prove that people deliberately copied something. Moreover, students may (or should?) be encouraged to work on the assignments in teams and hence, they may produce similar output.

But the dealbreaker for me is: At the place where I am students may cheat on homework assignments, but they will never be able to graduate with plagiarism and copying only. There are more safeguards: oral and written exams, one-on-one meetings with a thesis advisor to discuss the progress of the thesis, theses presentations… As a consequence: I am pretty sure that my students do copy homework regularly. I tell them that they miss a chance to learn something if they write down other people's work and that this will backfire in the written and oral exams later (and it does…). But I never investigate any case of plagiarism in homework (however, I do investigate cases of plagiarism in theses).