Could you expand on this? Where is the line? What's work and what's business?
From the US web site
participating in business activities of a commercial or professional nature in the United States, including, but not limited to:
Consulting with business associates
Traveling for a scientific, educational, professional or business convention, or a conference on specific dates
Settling an estate
Negotiating a contract
Participating in short-term training
Transiting through the United States: certain persons may transit the United States with a B-1 visa
Deadheading: certain air crewmen may enter the United States as deadhead crew with a B-1 visaIn none of those cases there weren't any kind of tricky questions asked at the border, I am sure I would have given the wrong answer. But I have been to more aggressive inquiries on conference trips, when giving the right answers was easy.
I have always assumed working means having a contract with an US employer (or of course self-employed).
One would assume a large corporation had processes in place not to expose their employees to such personal risks.
How much and what types of questions you're asked can depend on many factors. Whether the officer had a fight with his wife, your profile, how his day has been going etc. Sometimes it's "have a nice day" and sometimes it's a longer interrogation including secondary inspections.
This is a common misconception that work means having a contract with a US employer. As another example you're not allowed to work remotely for a non-US employer while physically being in the US.