The streaming part or the downloading/looking at code?

You can look at leaked source code for educational purposes in most places (not legal advice). As far as I understand leaks are commonly used in vulnerability research for example (if the bad guys can use it so can bug hunters).

Streaming copyrighted material is a separate issue - but using it for "criticism, comment, news reporting, teaching" should fall under fair use, no?

What's wrong with looking at public code? The code is public, regardless of how it became public - this isn't someone's personal life being exposed. If twitch is damaged by streaming this, it's only because their poor code quality is being examined publicly.

I can certainly understand why twitch banned this and don't blame them (although I think it's stupid), but I see nothing unethical about openly talking about this code in the public now that it's already there.