meg, how many of those people who SPEAK in the way you described would WRITE in the same manner? How many fewer would write in that way of if their paid occupation was writing?
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snm I think I see a point being missed here.

You told us that over one third of the content provided by the folks you employ who are nominally native English speakers is rejected by you. That is a matter of YOUR choice when making your selections of workers and your choice when you set standards. There are plenty of people who have English as a first language who are barely literate. I'd just not call them writers or employ them as writers.
If a person describes him, or herself as a writer then I'd expect, as a matter of course, fluency in the chosen language and the ability to correct their own work.
A person from China, the Philippines, France etc would be most unlikely to have English as a native tongue. So yes, as you noted, bad writers of English DO come from all over the globe, its something of a given.
We have examples on this very forum of people who want to attain higher standards of content output but who do not have good command of English, but who are likely pretty good at Mandarin, Tagalog, Spanish etc. Those people who read and post here are those whom I am addressing. Your employees whether from the USA or the Indian sub continent are, I am sure, not likely to be reading this forum.
In general though, if posting about this stuff here helps get slightly better content on my sites then I will be a happy man.

By volume there is a clear variance of source of poor content submitted to my sites. The issue is VASTLY in favour of China, Philippines and India. Those places are the origin of most of the world's low quality rewritten and spun continent. If one wanted to one could even make a good stab at particular cities, especially in India.

Look at your own stats on one of your UGC sites, you'll see the same picture.
snm, if you do not find native English to be of much consequence then you might want to consider your hiring policies because you are likely not employing native English speaking
writers.The bottom line is this: as a publisher of user generated content, we reject most submissions, currently around 70%. A decent proportion of those submissions would likely have been accepted had they not been badly spun. Spinning, while a very, very useful tool has, over the years, been the single biggest contributor to quality decline on the internet.
A very large proportion of ALL submitted content that I see comes from, in this order: India, China, Pakistan, Philippines. In many cases, it is clear that the ultimate client is from the USA or UK given the links in the articles.
I have clients who have open access to my sites because they produce consistently good quality work. I don't think any come from anywhere that English is not the first language. I really wish that it were not so.