Let’s say you own a small business, maybe a salon. You managed to pay salaries and rent and buy disinfectant, plexiglass, and disposable masks and gloves when you were closed for COVID-19. Now that you're open, you're doing your best to keep staff and customers safe.
Then Customer K walks in with no mask. You offer a disposable.
"You can't make me wear that," she yells. "I want to see the manager!"
When you ask her to leave, she turns her head and departs with a final threat, "You’re violating my Constitutional Rights! I'm going to sue!"
The First Amendment (in fact, the entire first ten Amendments--the Bill of Rights) protects people from the government. The government cannot (usually) prevent you from speaking, assembling, and practicing your religion. But the First Amendment does not reach a private commercial business. The beverage store can require a shirt and shoes, a restaurant can say no smoking, and a bar can refuse to over-serve.
"But I have a condition!" she argues.
Federal statutes (like Title III of the Americans with Disability Act) require businesses to provide equal enjoyment of goods and services to customers with disabilities. But a business need not accommodate someone if doing so would be unsafe. Current CDC guidlnes recommend masks indoors within six feet of another person. Allowing unmasked customers could create a health and safety risk to employees and other customers. Especially since Covid-19 can be spread by asymptomatic people and those who have no idea they have the virus.
Business that require masks may lose mask-refusing customers. But allowing mask-refusers to put staff and other customers at risk is a valid business decision.