Press releases

Once they try to fit themselves to some arbitrary style guide written by people who actively despise the actual language (say, at any major newspaper, or the communications department of a major corporation) they will no longer be able to generate natural-sounding sentences, and among the unnatural results it's much harder to spot actual errors.

You might think "But, surely there are rules" and there are. But just because there _are_ rules does not mean that ordinary people using the language are conscious of what those rules are, any more than they could articulate the rules by which they walk around or catch a ball. Many smart academics have put decades of work into trying to deduce the rules by experiment, and they have made some progress. You can see a good guide to what we knew as-of the turn of the century in the book "The Cambridge Grammar of the English Language".