The letter is addressed to GitHub leadership. If Microsoft does not allow them to terminate a contract for widespread ethical and legal violations, they should quit.
The CEO of GitHub is not going to starve if he gets fired.
If the CEO of GitHub gets fired it should be because he mismanaged communicating that GitHub shouldn't be the moral arbiter its customers. I think GitHub leadership's morals align with Microsoft's here. I also share this moral framework, I don't want private companies creating an extra-judicial layer of subjective morals. You shouldn't pick and choose who your customers are for many reasons.
Go for it! I didn't say it was an inappropriate channel (that was a GP post). I do think it's misguided and against the spirit of open source software though.
Does it make it anymore acceptable when a cashier is robbed or shot and killed on duty that the store is located in a high crime area? Jobs will be filled by someone. That never excuses abuses towards them.