There's spoken English, and there's written English, and they have different rules. And the best songs are spoken English. (I mean the best English language songs - but the same is true of other languages. Geiorges Brassens' French isn't exactly Academie Francaise French.)

Though in fact split infinitives are perfectly OK in written English too, as my 1926 Fowler's Modern English Usage confirms. That's early 20th Century. Mid 25th Century too - you can just bet Jean-Luc Picard has a copy of Fowler's on the Enterprise, as he continues to boldly go...