Discrimination is not illegal except when it is discrimination against particular protected classes -- federally, this includes race/nationality/citizenship, religion, sex, age, disability, or veteran status. A private company is free to discriminate on the basis of political belief.
That's not to say it is right or acceptable, but it isn't illegal.
There are some states, like CA, where discrimination based on political views is illegal, but it’s practically-speaking not too hard for an employer to find a more traditional performance reason that may be related to political views. Caveat: IANAL
Not all types of discrimination are illegal - in particular, the GP is stating that refusing to do business with people of a certain political leaning is not illegal.
That's not to say it is right or acceptable, but it isn't illegal.