Additionally, technology changes all of the time: new screen readers and new web APIs come out and and more. Best practices don't necessarily get perfected on day 1. Do you expect every website to get continuously rewritten just to keep up with the latest opinions on what's best? This is complex enough for large tech companies who can write a blank check for a large team of full-time developers who can work full time on nothing but accessibility, let alone a little corner cupcake store who managed to save up enough to build a custom cupcake website or something. Do we seriously expect every small non-technical business eking out a living with a small store to be experts on every facet of accessibility?
If we're going to cover websites under the ADA, I think there should a lot more leeway for "reasonable accommodations" that can be made. If a small business can't make an accessible order form for some reason, they should be able to take orders over email or the phone or something before getting sued for this.