You are making a large assumption here. I have never seen this be explicitly forbidden in any syllabus nor student conduct code.
You can do a search of their "Honor Code"[1] for the keywords --- "online", "post", "distribute", "share" -- none are even mentioned.
I'd like to be proven wrong -- but this just seems like a fallacy. Posting code online is fine -- copying it and passing it off as your own is not.
[1] http://cs.illinois.edu/current-students/undergraduates/polic...
Laws (and I'm considering honor code policies to fall in that category) are not programs. They intentionally have some ambiguity so that humans can interpret the meaning. Here, the intent is clearly that students should not share their work. They need not explicitly forbid all forms that such sharing could take.
"As described in the Student Code, aiding someone else's cheating also constitutes cheating. Leaving your program in plain sight or leaving a computer without logging out, thereby leaving your programs open to copying, may constitute cheating depending upon the circumstances. Consequently, you should always take care to prevent others from copying your programs, as it certainly leaves you open to accusations of cheating."
Students are definitely not forbidden from using version control. In fact, version control is encouraged. What they are forbidden from doing is making their repositories publicly viewable.