Civil rights laws prohibit the Federal government and others from firing someone based on race, so, yes, firing a USA because he is black is illegal. Likewise, Bush would be prohibited from firing USAs based on religion or gender. I am aware of no such legal protections for people fired because of their failure to be good Republicans. So, I don't think it is illegal for Bush to fire USAs for partisan political reasons, but it would be unethical, immoral, and, in an extreme case, the sort of thing that could justify impeachment. I'm sure I will get ridiculed for that last part, but I am not advocating that anyone be impeached over the USA thing. I only mention it to help draw the distinction between acts that are illegal versus those that are improper. Even though it takes a "high crime or misdemeanor" to warrant impeachment, I believe that term has been interpreted to include abuses of power that are not technically illegal.