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.
If you have power, you are responsible for what happens with it. It's not just free money.
Cutting off GitHub access to ICE engineers is like sitting outside their offices banging pots and pans together for a few weeks. A lot of people are generally worse off for a short while, GitHub revenue goes down, maybe they lay off a few engineers who were supporting that customer, a few news articles are written alternatively praising another step towards corporate activism or bemoaning cancel culture.
The most important part of convincing GitHub to cancel ICE to those who are rooting for it isn’t so much ICE losing access to GitHub, but another drop in the bucket toward normalizing the politicization and disruption of basic services to deplorable customers.