No, pull requests are not illegal, at least when done on Github, because by posting code on Github (that you are allowed to post) you grant Github and its users certain rights:

https://docs.github.com/en/github/site-policy/github-terms-o...

> By setting your repositories to be viewed publicly, you agree to allow others to view and "fork" your repositories (this means that others may make their own copies of Content from your repositories in repositories they control).