Or just use Git without Github - Git is free and open source. Or use Subversion. All these protesters are actually doing is _hurting_ open source by removing a revenue source that could be funneled into improving other open source projects. Unless they’re going to follow this line of thinking to its logical conclusion of creating a special NGGPL (non-government GPL) license and preventing government agencies from using open source projects.
Is that not a reasonable end game for them? Some folks want to be able to say, "My work doesn't support ICE". If ICE has to move to an alternative, then those devs can continue to build and not support ICE.
Presumably they want to abolish ICE entirely, as has been widely called for.[1] Pressuring technology companies to not work with them is just one tactic in the movement.