Should call to actions (CTAs) be included in product search result listings?

In my daily trips around the web, I've noticed that many of the most popular ecommerce sites (e.g. Walmart, Target, Amazon, etc.) are removing CTAs (e.g. "add to cart") from search results -- and I can't help but wonder why.

Is there any existing research that indicates improved click-to-sale conversion rate by removing the "Add to cart" button from product listings?

As UX designers, have any of you made this change on your site? What was the perceived problem that drove the change? Did it work?