Of course there are people who believe that there's only one correct version of English, and that any variation on that, be it dialect, argot or whatever, is essentially wrong

There are people like that, it's true, but I think that we would actually find common ground in believing that they are kidding themselves. The thing is, dialects and argots are suitable to their circumstances; not to formal usage, which will be received by people outside that context. I consider that a sign used by a national supermarket in stores across the country to be formal, and therefore one that should be beholden to formally accepted standards of grammar and spelling.

Obviously there are times when words come into the 'core' language from the fringes as they do from other languages, but just because that is the case, it doesn't mean that no one should point out when something is wrong by the current standard, even if a lot of people don't realise it is wrong. I might even go as far as to say that the more people there are who don't recognise the distinction, the more reason there is to draw attention to it!