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.
If you would've spoken out, then you agree with the principle but don't agree that ICE is "bad enough" to warrant this treatment. If you wouldn't have spoken out but wouldn't have worked for them, then you agree that working on these systems is clearly unethical (and thus IBM was acting unethically) but feel that ethics are less important than not disrupting the freedom of a company to sell their services to whoever they like. If you would've worked for them and wouldn't have spoken out, then we have very different views on ethics and I'm not sure we're going to agree on anything.
Yes, laws should be changed but businesses should be held accountable for who they do business with. You'd better believe that the US government wouldn't have the same rosy outlook you do if they discovered that GitHub was selling software to known terrorist groups.