Comment by petercooper

generally speaking, countries don't want you to be in them and be paid by a foreign company.

Curiously, the US prefers this for temporary visitors. If you are being paid by a company outside the US to speak at a US conference, it's fine, but you are not allowed to be paid by a "US source". I guess the theory is being paid by a non-US source means you're "on business" rather than "working in the US".