Because if they do declare all sales final, it kills sales. Customers do not like final sale, and regularly go for sellers they trust to take care of them. Generally final sales are only acceptable to consumers on things that obviously have logistical, regulatory, or cleanliness concerns.

If return windows were shortened via legislation, it would massively advantage shadier resellers, as nobody would be able to offer better return polices to offset a higher end price.

That's not how the legal theory works, fyi. The theory is that “sincere belief” is not a choice, and since it’s impossible to determine that externally the only choices were to not protect sincere belief or to protect all religion sincere or not.

The reason why the “bake a cake” thing was ever an actual point of controversy is because it centered around an immutable aspect of people; their sexual orientation. There is leagues of difference between denying service due to who they are vs. their behavior.