it is, for private businesses at least. Decades ago the US Supreme Court has recognized a fundamental freedom of association that allows businesses to choose who they do business with as long as they don't discriminate against certain protected classes. As long as you're not discriminating on race, color, religion or creed, sex (including pregnancy, sexual orientation, or gender identity), age, disability, veteran status, genetic information, or citizenship, then they're good to go. People who are hesitant to get vaccines (without other mitigating factors like legitimate religious objections) are not a protected class.