The visa system in very strict in the US. It is illegal for any non-nationals to work without a visa in the States, and for a normal employment visa within a company (L-1 visa) it is required for your employer to recommend you. This has to be justified for management or specialist skill reasons, so isn't just handed to anyone. This visa offers between 1-7 years of labour depending on your original country of residence and skill levels. The visa is also expensive ($300 base cost, followed by a number of additional charges that bring it somewhere up to $2000-4000) and a slow process.
Short term work (no longer than 6 months) can also be handled with a B-1 visa. Again, this costs money, and needs at least 2 months notice of arrival (ie, must be sent 60 days before you will be arriving to work).
As you can see, for an international company that is likely to be sending you between offices at short notice, this is an expensive and slow process. If they need expert X in their New York office next week to complete task Y, and then back in their Paris office on the Friday, it is helpful for them if the expert already has free right to work in the US. This could be through marriage, past residency, greencard lottery (not available to richer nations as far as I'm aware) and a few other smaller visa groups. Otherwise, they will need to find a similar expert in the US, or plan international long in advance and at considerable cost to the company.
EDIT : Based on Scaahu's link, it is in fact up to 6 months (rather than 3 months/90days) that one can work in the US on a B-1 visa. Practically speaking this does little to change to direction of answering this question, but I'll make it clear here in case anyone else finds the link useful.