Except it hasn't. The U.S. legal system is based on precedence (common law), and other than a scattering of local Title III judgements (Long v. Live Nation Worldwide, Inc., Haynes v. Hooters of America, LLC, Winn-Dixie, Blick Art) there is no concrete or shared understanding of how ADA applies to e-commerce.
I welcome all these lawsuits, for no other reason that we'll reach a consensus sooner-than-later on compliance.
Pretty sure the law doesn't say anything in particular about websites.
I don't usually find myself scratching my head at SCOTUS. Full and equal enjoyment of the goods and services is the pizza, not the website. The website is just one means of obtaining said pizza.
As I said in another comment about another case that didn't make it, the acceptance rate of the Supreme Court is single-digit percentages. Don't overinterpret them not accepting a case. They have to not accept a lot of cases. It may not be entirely meaningless, but it isn't very meaningful either. There will be another case about this in the future if they do choose to take it up. I'm sure this is a perennial topic.