The answer is not clear cut. It depends on a number of factors, including whether the business's reasons are political, moral or discriminatory, and the person claiming discrimination falls under a protected trait or class, such as race or religion, lawyers say.
With specific federal law and state laws that vary to certain degrees, it also depends on where the incident happened.
Under American law, a business owner has the right to refuse service to some customers. But federal and a lot of state laws say you can't discriminate against customers based on factors such as race, religion, sex or national origin, said Alexandra Brodsky, a civil rights attorney at the National Women's Law Center.
Turning away Sanders because she's President Trump's press secretary gives the impression that public spaces and public businesses are not open to all, says David Cole, legal director at the American Civil Liberties Union. However, he notes, a business being uncivil or rude isn't illegal.
In the District of Columbia, there's a list of 20 protected traits that include race, religion, disability and political affiliation.
If Sanders was refused service in DC, and it was considered a result of her political affiliation, that would have been illegal in the nation's capital. Virginia has a similar list, but it doesn't include political affiliation.