>>508621815ESTA was honestly strick.
Reading it we were actually in a legal gray zone, because we didn't worked in the US, we were just traveling to where we had to work later, but still, it was funny that only the guy who had the normal VISA was under scrutiny, they didn't even ask anything from him, just checked the data on him and maybe the bags more closely.
Although all the working gear/tools (5 extra big ~20-30 kg bags) were with him, so maybe that was the thing they found strange.