The moment I saw your title, I wanted to give you a huge "Amen!" When I read the first line of your article, I wanted to laugh out loud because it doesn't get more real than boob sweat.
The reason those editors don't want to even look at writers from certain countries is because editors like that are lazy, and those writers' English is often poor.
Sadly, I have seen the same here on Medium--writers who don't know how to make verbs agree in English--or they get the prepositions wrong, or they make plurals of collective nouns, that sort of thing.
These errors can improve with time and practice. Few editors, though, are willing to read through articles filled with grammatical errors, even if the content is otherwise good. Good mechanics (grammar, punctuation and spelling) are considered basic, and a writer who cannot do the basics well will not be considered.
In the writing world, being a non-native speaker of the language you're writing in is godawful difficult, but it is also no excuse for poor grammar. Foreign-born writers are expected to adhere to the same quality standards as native speakers must.
True, many foreign-born writers speak and write fluently in languages they are not native speakers of--far better in those languages than I could speak in their native languages. Those writers ought to be given an equal chance with native speakers to get published in whatever publications they desire.
If you can't tell that a writer isn't a native speaker of the language one's publication is written in, then that writer deserves consideration because that person clearly has the drive to excel and is worth listening to. I agree with you; there should be no blanket rejections based on country of origin.
Regarding content mills--places that pay very low for long articles--No one should write for them. Professional rate is $0.08/word USD in the United States, according to the Science Fiction Writers of America (SFWA). That is considered the very barest minimum that you can be paid and still afford to pay rent and put food on the table. The next time someone offers to pay a measly $5.00 for an article, that customer should be told that that amount of pay will get them 62 words.