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.
This is the reason why I think web design firms have to position themselves as solution providers and not just web guys/gals. A client doesn't need a website, they need sales. The website is just part of that solution but it's not the solution and unlike web design, service like email marketing automation, SEO, and PPC requires an experienced professional.
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.”
The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description.