[duxup]

In my experience it is usually for legal reasons. In those places the employer knows the lay of the land as far as employment law and an a independant and predictable legal system that is also understood / preditible.

I worked for a company that had some major banks and financal institutions. Anyone working on their systems had to be a US citizen, living in the US (even the european banks required that at times).

I was actually involved with one customer who I happened across some strange monitoring setups and it turned out it was the remnants of a company they used that was based outside the US / Europe when the company was expanding quickly. This outsourced company could not explain why they did what they did and after an internal investigation / consultation from legal they realized how little leverage they had with this outsourcing company (pretty much none). It could have been incompetence, but from what I saw the setup was far from something you'd do 'accidentally' or something you would do to accomplish another goal. After that they reviewed a lot of their outside companies and laid down a lot of new US (and sometimes some local exceptions) only rules. Fortunately for my employer they had already established those rules for similar reasons / other customers had demanded it.