Thanks for your comment and sources. I've found similar information already. What I was really looking for was, say, an in-depth legal analysis of B1 visas and visa waivers.

But as far as I can tell now, there's not any really clear definition of what counts as work under those visa (waivers), and the ambiguity was tolerated by all parties. So, in effect, the only reasoning is (like always) "this is illegal because we say so".

Thanks again for the info.