Actually, no one has made any implied suggestions at all. It is understood by most that if English is not your first language, mistakes in grammar and spelling are inevitable. Hell, it's hard enough for those who only speak the language to always write and speak error free.
Maybe, but funnily enough, in the quark and the jaguar Murray Gell-Mann makes the exact opposite point. He notes that there are foreigners who are more obvious by the fact that their grammar is "perfect" rather than because it isn't. I agree with him.
Although the reverse is true as you note above. I don't think they [in this context, as I said, which meant "this forum" if it wasn't clear] are particulary different from the native English who can't either. Few have come here to learn English afaict. So I can't see that as a valid reason. If they have, they'd be better finding somewhere else to learn from, until they have the ability to tell good from bad.
At some point they are going to have to read, listen and comprehend both, at least to communicate with a lot of native English communities [or if you prefer, foreigners and kids that have read the wrong posts

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Similary there were several references to youngsters but there are plenty of old people who can't spell and don't use correct grammar. Again, in this context, there are few here, if any, at an age where their age is the reason for their bad grammar and spelling. Nor are they so young that we need to worry about our spelling and grammar for their benefit. Although I'd concur in some contexts that'd be a good idea, I have a 6yo if I want to worry about teaching children

So unless the OP is both foreign and young and wishes to learn and thus speaks for his own desires rather than from his own prejudices about who can and can't write properly,, I see implications in that post, even if you don't

Plus the point of that post seemed to be that if you can do it then you should. Which implies that there are some who can but don't. [If this implication wasn't there, it would have been pretty pointless to demand it, since otherwise, everyone who can must already be doing it anyway. Although I haven't spotted who they are yet

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I concur that some don't do it out of choice rather than ability, and some of those are neither English nor old. If they should do it, then I think you would need better reasons than those given. Because it would be telling them to write properly so that they can teach themselves something they can already do
