I might be wrong, but I don't think any of these 3 "rules" are formal rules of grammar. They're guidelines to be followed most of the time but broken when appropriate. The split-infinitive snobbery comes from Latin, in which infinitives are one word and can't be split, but it doesn't make sense for English (although infinitives should be split rarely).
These are rules as much as many others are. You won't go to jail or be a literary outcast, so you're right, like ALL grammar rules, you can break them.
That's why the critique of S&W the other day didn't make any sense. S&W is a book on style.
