No private company is obligated to accept a contract from a federal agency. To expect them to do so would be the inverse of the supposed freedoms we supposedly value in America. Specifically the first and third amendments to the Bill of Rights, but that's just scratching the surface of how un-American that would be. Worst case, the government can set up its own infrastructure, and that probably wouldn't be a bad thing anyway. Some people talk about privatization of government resources as if it's an inherently good thing that saves taxpayer money, but if anything the last 40 years has shown the opposite, especially WRT to the likes of Blackwater and private prisons. In many cases privatization is more expensive and simply serves to put public money in private hands without oversight to reward corruption.
Also it's misleading to refer to it as a "political reason". There are two different things people mean by the term "political". One is through the lens of party tribalism. Like, if a restaurant owner put up a big sign out front that said "I only serve registered Democrats, the rest of you can GTFO." That's about the shittiest and most indefensible form of "bringing politics into it" that I could possibly think of. Probably wouldn't be illegal but obviously I wouldn't think too highly of the restaurant owner and I would hope he quickly went out of business because everyone (left and right alike) would think he's a douchebag.
That's one meaning of "political". The other meaning is simply, "reacting to current events", independently of tribalism or political parties. If someone is actually acting based on moral convictions rather than party loyalty or tribalism, that's very different. I realize that an anti-ICE POV and pro-immigrant POV is strongly correlated with Democrats and the opposite is strongly correlated with Republicans, but if we start casting every single moral conviction or political belief in terms of party loyalties then we have freaking lost as a nation. Party-based tribalism is always bad, full stop, but people acting on moral convictions are as good as the coherency of their moral convictions.
Like, if the US government started rounding up folks by ethnicity and putting them in camps based on obviously racist motivations, like they did to Japanese-Americans in WW2, and it was based on bills passed by Democratic politicians, and the Republicans, bless their hearts, decided to fight tooth and nail against it because Democrats, I would honestly hope that all businesses, regardless of their owner's or employee's political leanings, would ignore the party politics aspect of it and only make decisions based on moral convictions. The parties fight each other all the time but we can't fall into the trap of thinking that political parties are anything other than an unfortunate reality of our system of government. Many of their conflicts are fights they pick with each other to rally their base, whereas if they were actually interested in responsible governance they would try to find common ground and try to negotiate something that actually helps the country.
Some people just have a tribal mentality though. They can't formulate thoughts on any issue through any lens except that of party politics. They think of themselves as "this type of person, not that type of person", and their first cut at bucketizing the population is lumping them in with one party or another. That's not democracy, that's about the stupidest freaking tribalism imaginable. Keep in mind that it wasn't until 1994 (or 1996?) that "bipartisan" and "compromise" became dirty words in political coverage, specifically to the Republican Party thanks to Newt Gingrich, but to a lesser degree to the Democrats as well as they circled the wagons in response. It's only been the last 26 years that parties themselves have become more important to some Americans than America itself, or Americans as a whole.
Besides, what possible negative consequence could GitHub suffer from this? Besides losing the ICE contract? Does anyone seriously think that other government agencies or companies would be so butthurt about GitHub's position on ICE, that they wouldn't want to do business with them? That only makes sense through the lens of the two political parties defining the totality of what it means to be an American, and if that's where people are at, America deserves to crash and burn because it's lost whatever soul it may have once had that elevated it. George Washington's farewell address was on this very topic. He thought the poison pill of the Constitution was that it set up a system whereby people would naturally form voting blocs, and before too long voting blocs would coalesce into two diametrically opposed political parties, and then the system (and America) is screwed. Which is exactly what happened. But it doesn't have to be that way. But the election process itself would have to be reformed into a less antiquated, non-FPTP-based system.