This is the point though. As a client I don't want to throw 400 errors. As a server I don't want to throw 500 errors nor some 400 error.
As an example, if either client or server sees a spike of 404 errors they want to investigate. When the result of that investigation is "some crawler went haywire" or "a user is trying to access resources that don't exist" it's annoying. So the 200 OK with an error is an attempt to stop those sorts of scenarios. Of course like anything people take it too far but there's decent logic behind it.
