@Setanta,
Quote:who therefore don't speak standard English
My take on this is that there is not a single correct accent of English. There is no neutral accent of English. All speakers of English need to cope with many different aspects and learn how to understand them. Some accents are associated with social groups who have high prestige (the kinds of accents spoken by highly educated people, for example), but there are also many of these high prestige accents, all of them regionally based. The accents that are traditionally taught to non-native speakers of English are high prestige accents from various places.
The two most commonly taught accents (in the world as a whole) are both rather artificial: ‘General American’ (more or less a Mid-Western and West Coast accent, and used by some high prestige speakers outside this region too); and the British accent ‘RP’ (which developed in the private boarding schools of the nineteenth century, and is associated with high prestige groups in England). Both these accents are used over a wide geographical area, though in world terms both are regional accents (General American is a US accent, and RP is an accent of England). They are heard more, by more people in the country, than are accents which are associated with a smaller area: so people are familiar with them. These accents are the ones transcribed in dictionaries. Because they are used over wide areas, and used by people of high social class, they are seen as being suitable to teach to foreign learners of English. For this reason, they are called ‘reference varieties’.