It's the law in many places.

United States Laws that Govern Access

• 1990 Americans with Disabilities Act (ADA)
  Covers commercial companies, non-profit and private organizations as well as all units of government except the federal government which require "reasonable accommodations" for people with disabilities.
• 1973 Rehabilitation Act
  The federal government is covered in the 1973 Rehabilitation Act which require communications with people with disabilities to be "effective."