The first job of an editor is to ensure proper grammar. The SF Chronicle should go down in flames for letting in split infinitives. Probably they just want Saunders to look bad.
I would have, until recently, agreed with you — that's certainly what we were incessantly told in grammar class. However, the rules have been heatedly debated; and are gradually being eased.
It seems that the rule originated in an attempt to import Latin rules of grammar. Since Latin infinitives cannot be split (being single words) — it followed that English infinitives should not be split. Or so went the reasoning.
I have a genuine fondness for our language, but I sometimes have been known to intentionally and flagrantly split infinitives.