Sooo, what you're saying is that it's OK for any company to ignore any rights of yours that they don't agree with? Interesting. Of course, it isn't illegal, but does that still mean they should do it?
Exactly. They're a private company, they're free to do whatever they want within the legal limits. The argument isn't whether they're allowed to do it (whether the first amendment applies here), but rather, should they do it?
I say probably not. In many cases, morals/ethics are subjective and should not be enforced upon others. The key word is enforced -- I'm all for them contacting the user and saying "Listen, man, this isn't cool -- please change it."
In general, I'm against the enforcement of personal morals upon others, except in obvious circumstances (we probably shouldn't hurt/kill/steal from/etc. each other). Discussion and debate is cool, enforcement is not.
And just as the company itself has the right to censor whatever it pleases, its users have the right to stop using their services when they feel that the company is infringing on their rights -- which is what a lot of people here are saying (they're not saying "this is bad, the company should get in trouble for it," they're saying "this is bad, I refuse to do business with a bad company").
open source projects are certainly not paying github