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.
If we can’t uphold standards in formal written language, it is little wonder we can’t uphold standards in behavior.