Andrej Bjelakovic asked (in this thread) “what’s the deal with the ‘I can’t help but’ + bare infinitive? Is it frowned upon only by some fuddy-duddy prescriptivist or is it generally considered non-standard?” The short answer is that it’s fine but it has been frowned upon. The long answer follows.
As always in such matters, I turn to Merriam-Webster’s Dictionary of English Usage (or its equally reliable, cheaper, and more available twin, Merriam-Webster’s Concise Dictionary of English Usage; in this case, the former was closer to hand). The entry begins as follows:
cannot help, cannot but, cannot help but A lot has been written about these phrases. To put as charitable a light on the matter as possible, most of what you may read is out of date. We have hundreds of citations for these phrases, and we can tell you two things for certain: these phrases all mean the same thing — “to be unable to do otherwise than” — and they are all standard. To the usual three we can add can but and cannot choose but, which also have the same meaning but are less frequently met with. We will take up each of the five in turn.
They say can but was called “pompous” by Bernstein but give examples where it sounds “natural enough”; they point out that cannot choose but is often used with a conscious echo of Coleridge’s “The Wedding-Guest sat on a stone:/ He cannot choose but hear.” Cannot help “is grammatically the odd one of the five. It is followed by a present participle, whereas the others are followed by the bare infinitive.” Their first citation is from Swift (1712): “yet I cannot help thinking, that . . . our Conversation hath very much degenerated.” Cannot but “is an old established idiom. It has even been a favorite of some of our old warhorses of usage — Henry Alford, Richard Grant White, Fitzedward Hall…” They give citations starting with George Farquhar‘s 1698 Love and a Bottle: “I can’t but laugh to think how they’ll spunge the sheet before the errata be blotted out.” Finally we come to the usage Andrej asked about:
Cannot help but, which may have been formed as a syntactic blend of cannot but and cannot help, is the most recent of the phrases. It appears to have arisen just before the turn of the 20th century. Three sources — the OED, Curme 1931, and Poutsma 1904-26— all give the English novelist Hall Caine as the earliest source. Two of his novels, The Manxman (1894) and The Christian (1897) are cited. We began to acquire citations in the 1920s, and a great many from 1940 on. [They give a dozen citations.]
Only cannot but and cannot help but have been the subject of much criticism. Vizetelly 1906 warned readers to distinguish between can but and cannot but — as if they meant something different; Bierce 1909 condemned cannot help but; Utter 1916 — the only one with foresight — recommended simply accepting can but, cannot but, and cannot help but as idioms. A great many other commentators have had their say, many of them finding fault with one or the other by resorting to logic — their own brand — but of course logic cannot measure idioms. Degree of formality appears to be determined not by the phrase but by the choice of cannot or can’t in the phrase. You can use whichever one seems most natural to you; all are standard.
So there you have it. People will complain about anything, but that doesn’t mean they’re worth listening to; and logic cannot measure idioms.