Because that code isn't publicly available, just like every private repo on GitHub, and any other company's/person's proprietary/private code?
Again, if I put code in a public repo on GitHub and don’t include a license that allows it, you cannot copy that code. It doesn’t matter that you’re able to access it.
Cannot is a strong term. Telling me there may be consequences would be more accurate.
I can claim fair use and short of an injunction there's nothing you can do to stop me.
Claiming fair use is not a fix all card you can play like that, there are a lot of nuences for that to hold on trial.
I'm allowed to read it and learn from it. To distill the core concepts and recreate it.
You as a human being, not you as a for profit company building a for profit product.