Also visas, almost anyone on an employee sponsored work visa is not permitted to undertake contract work.

Also people with no visas. At the point in time we're hiring, many of the people we're talking to don't have the legal right to work for us.

that's not true if they are in their home country.

That's true for a small group of country pairs, and also depends on any corporate structure the interviewee has set up. (Most have none set up, which makes it more difficult.)

it should be true for the majority of countries in this world that allow their citizens the freedom to do contract work, whether with a local or a foreign company. some may require registration as a sole proprietor of some sort, but not all. the primary difficulty is figuring out income taxes.