This is a bit disingenuous. The sales boost from this Apple feature is almost certainly temporary and there are no guarantees that Apple will do such a promotion again. Also note that most of the apps in the store with VoiceOver support were not featured in this lis and received no benefit at all from their investment here.
On raw monetary grounds, investing in VoiceOver support is hard to justify on a raw cost-benefit analysis. I haven’t seen it materially affect sales under normal circumstances. Luckily, very little work gets you very far with VoiceOver conformance. It’s a nice thing to do that makes your apps accessible to an even larger base of people. As a developer, taking some time to do this is incredibly rewarding in ways other than bottom line profit. I love getting feedback from blind users of my apps. It makes me feel good that I’ve helped someone. That’s what makes adding VoiceOver worthwhile.