Any business that is considered a “place of public accommodation” is required to provide equal access to services under the nondiscrimination requirements of Title III of ADA. When you look at the guidelines closely, this includes hotels, entertainment venues, legal and accounting firms, retail stores, and virtually every business that is not a private club, including businesses that exist solely on the web.
I recently finished a training in ADA compliance for websites. It was illuminating and daunting, as I realizated that there’s a lot of work to be done. It also reiterated—in no uncertain terms—a gospel I’ve been preaching for several years now. Sites need to be accessible for everyone on every platform. Previously I was just focusing on responsive sites (sites that reformat for the device used, such as a phone or tablet.) Since responsive sites are de rigueur these days, I’d like to focus on your site’s accessibility…can it be accessed by people with disabilities? This includes screen readers and other technology. It’s not just a nice thing, it’s a civil law.

That’s good news if you are one of the many Americans who have a visual, hearing, or mobility disability that makes it difficult to access some information on the web. If you are a business owner who hasn’t made provisions to ensure that your website and other online assets are ADA compliant, you could be looking at a host of legal and financial penalties.
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.
You will also see tons of ” free analysis” offers out there. The truth behind them is they don’t even offer the service themselves. With all the hype of ADA compliance and websites, there are tons of people trying to take advantage of businesses that don’t know any better. Here at GrowIT media, we have trained staff who have brought federal websites up to code and understand how to do it properly.
According to Creating an Accessible Presence for the Lodging Industry by the American Hotel & Lodging Association, “Many businesses don’t understand that the law actually requires them to make their websites accessible to individuals with disabilities. There is a common misperception that the Americans with Disabilities Act and similar state and local laws only deal with physical buildings and facilities, so-called “brick and mortar” establishments, and that non-building-related business operations—such as websites—are not covered by these laws. Nothing could be further from the truth—at least with respect to websites that have some sort of connection, or “nexus,” to physical places of business.”

Yes, in time non-ADA compliant websites will be penalized in the search engines. It's already on their radar. But for now they are still negatively affected, just in an indirect way. Live traffic behavior (user experience) is now factored into Google's algorithm and is a ranking signal. Negative user interactions absolutely have negative affects on rankings.
The ADA statute identifies who is a person with a disability, who has obligations under the ADA, general non-discrimination requirements and other basic obligations. It delegates fleshing out those obligations to federal agencies. The agencies issue regulations and design standards. The regulations have the details on the rights of people with disabilities and responsibilities of employers, state and local governments, transportation providers, businesses and non-profit organizations. The design standards specify how many entrances need to be accessible, how many toilet rooms and the design for those elements.  To know what the ADA requires, you need to read the law, regulations and design standards.