Even if you don’t care about the ethical reasons to make web content accessible to those with disabilities, as an author you should care about the legal reasons. From a legal perspective, US courts treat an inaccessible website the same as an inaccessible physical location. If you sell an inaccessible theme, your customers are at risk of being sued.
There’s a top-selling theme that’s grossed over $11 MILLION for the author and Envato and it’s completely inaccessible, and not just to the blind. That one theme has put over 200,000 Envato customers at risk and it’s just the top of the iceberg. Themes like this give all Envato authors a bad reputation.
I did a quick search and it looks like MOST themes are inaccessible. I worked for Envato long enough to know how seriously they take their values. I’m sure that when they understand what a legal and ethical nightmare this is, they will do something about it. Until then, authors should all make a serious effort to protect their customers.