I know what you were talking about. You misunderstand their role fairly deeply. There's so much wrong with the idea that they would make Github enforce anything in this instance that it really is quite hard to start. It's just wrong. Entirely.
But go ahead and prove me wrong. Report that MS/Github is providing support to an anti-Israel boycott and therefore should no longer be permitted to export and should no longer receive tax benefits. They have a hotline.
In the US a blanket anti-boycott law would be unconstitutional to enforce. Government contractors in some states (maybe federally) are subject to anti-BDS restrictions, but to my knowledge GitHub isn't one.
Also, GitHub as an organization is not boycotting Israel, one of the users of its site is. The law / requirement is to state that you (i.e. the gov't contractor in question) will not boycott Israel. But it doesn't say anything about your customers also abiding by that provision.
The antiboycott provisions of the Export Administration Regulations (EAR) apply to the activities of U.S. persons in the interstate or foreign commerce of the United States. The term "U.S. person" includes all individuals, corporations and unincorporated associations resident in the United States.
Conduct that may be penalized under the TRA and/or prohibited under the EAR includes:
Agreements to refuse or actual refusal to do business with or in Israel or with blacklisted companies.
Agreements to discriminate or actual discrimination against other persons based on race, religion, sex, national origin or nationality.
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I would say that discrimination clearly applies in this case, and individuals do have a responsibility under the EAR.
The person "discriminating" is the person that rejected the PR, which they did being subject to the laws of Iran.
Github neither agreed to refuse, actually refused, agreed to discriminate, or actually discriminated against the person proposing the PR.
Github aren't responsible for some code of conduct regarding whether or not a repo owner will accept unsolicted (or solicited) PRs.
In fact, Github is directly engaged in blocking users from NK, Syria and Iran under the US law. They have to, it's the law that has jurisdiction over Github.