Poorly designed websites can create unnecessary barriers for people with disabilities, just as poorly designed buildings prevent some people with disabilities from entering. Access problems often occur because website designers mistakenly assume that everyone sees and accesses a webpage in the same way. This mistaken assumption can frustrate assistive technologies and their users. Accessible website design recognizes these differences and does not require people to see, hear, or use a standard mouse in order to access the information and services provided.

Many government services and activities are also provided on websites because the public is able to participate in them at any time of day and without the assistance of government personnel. Many government websites offer a low cost, quick, and convenient way of filing tax returns, paying bills, renewing licenses, signing up for programs, applying for permits or funding, submitting job applications, and performing a wide variety of other activities.
Web accessibility addresses the needs of every website visitor to achieve an optimal level of usability and ADA compliance. Many people browsing the web have a permanent disability (visual, mobility or neurological impairment), or a temporary impairment such as a broken arm, broken or lost eyeglasses, and so on. Many baby boomers have aging-related issues that make using the web more challenging, and they’re not alone–about 20% of the U.S. population has an identified disability.
You are correct, ADA defines an “employer” as any person who is 1) engaged in an industry affecting commerce; 2) employes 15 or more full-time employees each work day; and 3) for at least 20 or more calendar weeks in the year. In the context of physical spaces, ADA would not apply to companies with fewer than 15 employees. However, courts don’t seem to have come to a consensus on what digital compliance really should look like. Because websites can be accessed anywhere in the country, small business owners might potentially face lawsuits in unfavorable jurisdictions. If you are a small business owner, I would recommend consulting with an accessibility lawyer and ask what they recommend.

Courts have essentially taken three positions when approached with this issue. Some courts take the position that the ADA applies to all commercial sites because the law was meant to protect disabled individuals from having a more difficult time than able-bodied individuals from doing business. That is why the website Scribd was unable to get a case summarily dismissed and ended up settling.