[ghaff]

>trying to work in the US without a visa.

Yes, but that is a case where they're getting paid in the US to work.

The one conference horror story I recall from a few years back was someone was going to speak at some small UK(?) conference and they were getting an honorarium or something like that. And they told immigration and I think were denied entry for that reason. But I've spoken at dozens of events for free and it's never been an issue.