Wait, receiving compensation from a US source while in the US on an ESTA is prohibited? Wouldn't that mean that even the activities I described as legit "business" - a remote worker coming to the US on an ESTA to meet their US colleagues and attend a trade show with them - would be illegal? (Heck, if we take your wording pedantically literally, wouldn't it mean even taking a holiday to the US on an ESTA while remaining employed by a US company is illegal, even if you're not doing any kind of work whatsoever?)