The last two in my experience are handled by treating the remote employees as effectively contractors. They'll incorporate as an independent company, and then bill themselves out for whatever salary they're on, taking on responsibility for their own taxes and other requirements.
Is that actually the case? As far as I'm aware in the UK at least its not that you're outright forbidden to work as a contractor under conditions that would otherwise be considered employment, you just have to pay tax on that basis rather than pretending you're really an independent company.
