They can't do that. That is illegal to discriminate workers because they don't know ASL. What's if that town attracts corporations such as Google or Boeing or Verizon that are known as "hearing" companies decide to relocate there and they will ignore the policy by hiring workers that don't know ASL? They will sue Miller or city for discimination for sure.
There are ways around the discrimination of hearing workers. If the business is owned and operated by a D/d person, they do not have to, by law, make accommodations for a hearing person who can't communicate in ASL. However, when the business is owned by a hearing person, they do have to provide accommodations for the D/d employees. So, in effect, it isn't illegal.