There are many article creation companies out there that will only hire US writers.
However if you're an international writer there are ways around this rule legally and easily. I am in Australia - so that means I'm definitely not American. But I do speak English as my first language and we're taught UK-English in school, so this means I can comfortably write either US-English or UK-English articles. There are significant differences in spelling and grammar usage between the two. Knowing both is a huge advantage.
I take my freelance business very seriously and the majority of my income comes from international sources (US, UK or Europe) so I did the right thing and applied for an ITIN in America. This means I've been allocated a tax identification number with the American IRS for tax purposes, which means I am able to work for US-based writing companies that insist on an American social security number or tax identification number in order to receive payment.
It's painful for me at tax time that I have several countries vying to charge me the most possible amounts of tax, but when all is said and done, I'm an Australian citizen running a freelance writing business using an Australian Business Number (ABN) and our dear Australian Taxation Office has first rights to charge me a ridiculously high amount of tax first and before anyone else.
It's also worth noting that Australians earn 1.49 times whatever the US rate is shown, so whatever might look like low paying markets to a US writer means much higher rates for an international writer, so this can be to your advantage. (e.g. $10.00 US per article means $14.90 per article for me).
This doesn't mean you need to be Australian to make the lower paying markets work for you. It simply means that knowing a little about how the currency exchange rates can work for you means you have the potential to increase your writing income quite a lot.
For example: if you're in the USA and you write for a UK writing site, then if you earn £10 this translates to $14.30 US (rates subject to change - check your current exchange rates on www.xe.com).
So if ever you catch yourself thinking that there's no way an international writer can compete in a US-based writing niche, think again. There's always a way to accomplish anything you need.