
Quoting
Taxing Matters
You are right to suggest that the ADA might apply, though it wouldn't allow the customer to just shop as usual without a mask. First, let's put aside for a minute that some people going into stores maskless and then claiming a medical reason for not wearing masks really don't have any such condition. We all know some of those people are out there and those situations provoke a lot of outrage that can taint a discussion on this.
There are some for whom that claim would be true. Under the ADA the store could not discriminate against that person just because he or she has that condition. And to the extent that the disability prevented the person from using the store's services like anyone else, the store needs to provide a reasonable accommodation for that disability. However, such an accommodation does not include actions that would potentially put staff and other customers at risk of an infectious disease.
As the ADA itself states: "Nothing in this subchapter shall require an entity to permit an individual to participate in or benefit from the goods, services, facilities, privileges, advantages and accommodations of such entity where such individual poses a direct threat to the health or safety of others. The term 'direct threat' means a significant risk to the health or safety of others that cannot be eliminated by a modification of policies, practices, or procedures or by the provision of auxiliary aids or services." 42 USC § 12182(b)(3).
If that customer is infected with covid-19 then not wearing a mask creates a greater risk of infection to other customers and employees. As it is impossible to know exactly which customers are infected and which are not it's reasonable for the store to assume that any customer might be infected and take steps to reduce the risk of spreading that infection. That includes insisting that customers wear masks. Hence all the government and health professional recommendations that people wear masks in public. A store is entitled to take that into account and bar the customer from wandering around the store without a mask so as not to create that increased risk to others. If the store can reasonably accommodate this customer some other way that would not pose the the risk of infecting others, however, it may be required to do that.
In short, a disability under the ADA does not mean the disabled person can compel the store to do whatever the customer wants to address that disability. But the store might have to do something to address the disability in way that allows the customer to buy from the store.