Of course, if writing for a British audience, don't hire an American!
But none of that is a tenth as annoying as writers who place the currency symbol to the right of the number. "You can buy a widget for under 100$."
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I swear, by my life and my love of it, that I will never live for the sake of another man, nor ask another man to live for mine.
I think what they are mostly trying to say is they want people who write with an American standpoint. This means verbiage, grammar, feel, relevance, attitude, culture or whatever you want to call it. We are in the age of awkward writing since the internet is so widespread. Leet speak hurts as well, so some international communities may have the wrong idea of how an English conversation/writing style should flow.
According to me, Regardless of origin, anyone can be a good writer. When it comes to speaking, they might sound so pathetic. But I have seen number of people who are proficient writers and horrible speakers. Just because English is not their first language.
It actually depends on the type of job you expect from your content to do. For example, if you maintain a small site with a few tips and tricks about web codes and other internet-based materials, even writing like a foreigner will be quite OK because readers will get the ideas.
Now compare this with another case in which you would like to prepare some instructions for household appliances and want to send them to the shoppers living in the US. In that case is it one hundred percent recommended to employ copywriting services done by natives.