There is no doubt about US companies having to follow US law. But this is an internationally operating company, which means it has to also follow whatever law might apply to whatever market they operate on.

GitHub, as any other US company, has a choice/freedom to stop offering services to customers outside the US market, if the particulars of providing those services causes them to violate laws in at least one of the jurisdictions.

Of course, US companies should be rightfully pissed, if the US government puts them in a situation where they can not (legally) operate abroad. But that's something they should take up with US lawmakers.

At the end of they day, they are still (most likely) operating illegally on a foreign market, even if they are unlikely ever to be substantially punished for that. The thing is, the US has a rather questionable track record of coming to the rescue, whenever a US companies get into trouble for (illegally) doing business abroad. Ironically, whenever another country does that (e.g. China) the US immediately have a long list of choice words an allegations at the ready. Long story short: pure hypocrisy.