Because that code isn't publicly available, just like every private repo on GitHub, and any other company's/person's proprietary/private code?

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.

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.