Because you're pushing the code to GitHub, so they need to enumerate their rights in terms of what they can do with it once you push it there. But if you publish your code to PyPi, the relevant ToS is the PyPi ToS, which has no such clause forbidding either PyPi or others from using the code how they'd like to (and as mentioned by other comments in this thread, the ToS actually explicitly grants others the right to republish the code).

Because most company's/people are (AFAIK) under the impression that training on public data will fall under "Fair Use" because it's substantially transformative, in the case that it isn't then you've already agreed to it on github.

It's a fallback clause, "fair use" is irrelevant if you've already given github permission to use it. By adding that clause you can no longer argue that it's not fair use to use the code you put on github after agreeing to their terms.

Out of my own curiosity, which clause in the GitHub TOS are you referring to?