Question: What I'm not clear on is whether the business-but-not-work restriction... 1. merely means that the purpose of your visit has to be business-but-not-work, or 2. requires that the visitor refrain from all "work" activities while in the US, even if they weren't the reason for visiting and would have happened anyway.
Concretely: suppose I am a programmer who works remotely from Europe for a US company. I visit the US to meet my colleagues face to face and attend a trade show with them (business, not work), but these activities don't take up 100% of my time. Rather than sit around idly during the downtime between "business" activities, I do some of my ordinary programming work, of the sort I would do back in Europe. Have I broken the law?