Oliver Kamm, Editor of the Times of London, blasts prescriptive (rules-based) grammar in this delightful essay in the Wall Street Journal.
That’s how scholarly linguists work. Instead of having some rule book of what is “correct” usage, they examine the evidence of how native and fluent nonnative speakers do in fact use the language. Whatever is in general use in a language (not any use, but general use) is for that reason grammatically correct.
The grammatical rules invoked by pedants aren’t real rules of grammar at all. They are, at best, just stylistic conventions: An example would be the use of a double negative (I can’t get no satisfaction). It makes complete grammatical sense, as an intensifier. It’s just a convention that we don’t use double negatives of that form in Standard English.