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.
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.