Which ethical rules exactly did they violate?

Here is the Honor Code from the university[1]

I don't see any mentions of the words "share", "distribute", "post", "online", "github", etc.

What I do see, is a lot of talk about plagiarizing other student's work and passing it off as your own.

The github repos did not do this -- other students did this. You punish the students who commit the act of plagiarism -- not the source. You don't ban a book because somebody plagiarized it.

[1] http://cs.illinois.edu/current-students/undergraduates/polic...