All corporations are political. By accepting the ICE contract previously it was political. Now by reversing they would be changing sides. They were already in the political fray.
Just a reminder that government entities like ICE are executing the current laws of the land.
If Github was to pull out, ICE could simply find someone else willing to sign a contract with them, except it will make things worse, as that other provider will not be as diligent or reliable (if it was just as good, it would have been picked in the first place instead of Github).
When you see an unjust law, it should be pushed to get changed. Back when gay marriage was illegal, it made more sense to push for its legislation, instead of providers refusing service to state governments where it was illegal. People need to protest, call their elected officials, sign petitions, etc. Most importantly, people need to regularly vote, not just during general presidential elections.
That has nothing to do with Github. As a customer, I want to be confident that my service won't get terminated for some arbitrary reason, as long as I obey terms of service and don't break any laws. Giving providers the ability to cancel my service due to random whims in their workforce isn't something that I want to see in tech.