Dot Wordsworth don't want no stinkin' double negatives:
I heard someone on the wireless last week use [the] construction, ‘He could not help but turn,’ and I was irritated in an unfocused way. Should it not be, ‘He could not help turning’ or ‘He could not but turn’?
The Oxford English Dictionary reminded its good readers that the Collect for the 15th Sunday after Trinity includes the words: ‘The frailty of man without thee cannot but fall.’ Since the days of Hall Caine, we’d be tempted to make it ‘cannot help but fall’. The OED also obligingly mentioned the parallel Latin construction non possum non. It is just this element of double negativity that sets users’ heads whirling and tempts them into nonsense. The liberal-minded Oxford Dictionary of American Usage opines that cannot help but ‘should no longer be stigmatised on either side of the Atlantic’, but it goes on to mock someone who said: ‘I cannot help from refraining myself to comment on Ms Gabor’s flagrant disrespect of the law.’ I suppose the speaker meant either, ‘I cannot refrain from commenting’, or ‘I cannot restrain myself from commenting’.
Certainly cannot help but has become naturalised more completely in the United States. A decade ago, The American Heritage Dictionary of Idioms listed it with the two other constructions on an equal footing, without comment. A Dictionary of Modern Legal Usage, published by Oxford, judges that cannot but be and cannot help being both ‘strike modern readers as stilted or perhaps even alien’, whereas cannot help but be is becoming ‘an accepted idiom’. Well, blow me! I’m stilted and alien. It doesn’t make me a bad person.
I cannot but agree that Dot is right. Nevertheless, although I would not write or say "He could not help but...", I didn't at first see anything wrong with it. In contrast, "may have" instead of "might have" has me frothing at the mouth. Not seeing a mistake is one step away from making it. It's a slippery wedge or the thin end of a slope - I'm all but convinced of that.