I don’t doubt that American business owners are free to practice the religion of his or her choice. No one is preventing them from attending religious services or celebrating holy days etc.
Perhaps the real question is whether American business owners have a right to impose their religious beliefs on employees. Its a tricky thing, since it is done in ways which few find objectionable e.g. being closed on Sundays or getting a traditional holiday off.
But in the US, most health care is financed via health insurance. Do employers have a right to limit the health coverage offered to employees based upon the employers religious conviction, which the employees may not share? The immediate concern has to do with the contraception mandate. But wouldn’t an employer also be able to eliminate coverage for blood transfusions ( if the employer is a Jehovahs Witness) ?