I'm not into the exact details of it. Needless to say I got dinged on it from my own company due to traveling (for the company) into the state and had a chat with HR about it and it was explained out to me what was going on.
My guess is that for certain companies following the letter of the law is more important than others and report as they should. The state of NY may or may not bother coming after any company or individual they can prove had people in the area working in some capacity. In most cases I'm guessing it's not worth the bother but for large companies (like Facebook) it may be. So they want to know where their people are working from because it has tax implications that they may be on the hook for.
Even if the risk of being “caught” is small, it is still important to understand risks. If one does get in trouble for some other reason, then suddenly one can find the book thrown at them for multiple reasons.
My guess is that for certain companies following the letter of the law is more important than others and report as they should. The state of NY may or may not bother coming after any company or individual they can prove had people in the area working in some capacity. In most cases I'm guessing it's not worth the bother but for large companies (like Facebook) it may be. So they want to know where their people are working from because it has tax implications that they may be on the hook for.