Both the idiomatic expressions “can’t help + but + verb” and “can’t help + verb + -ing” are acceptable, and there’s really no valid reason why the first should be preferred to the second. This is because aside from being different in construction, each conveys a different sense when used in a sentence.
The first form, “can’t help + but + verb,” is a formulaic rendering of the idiomatic expression “can’t help but do something,” which means to be unable to choose any but one course of action. Consider this sentence using that form: “I can’t help but cry.” Here, with the expression using the bare infinitive “cry” (meaning the infinitive “to cry” with the “to” dropped), the sentence is complete in itself, and the sense is that under the circumstances, the speaker is unable to do anything else except to cry.
However, when the form “can’t help + verb + -ing” is used, the sense is that the speaker just can’t prevent himself or herself from crying. When this form is used, some qualifying phrase after the “-ing” form of the verb is often required to make the sentence complete and make sense. In the case of the verb “cry,” for instance, we can’t simply say “I can’t help crying”; both the grammar and the semantics of the sentence would be flawed in that case. However, if we say “I can’t help crying all night,” “I can’t help crying out loud that I’ve been robbed,” or ““I can’t help crying after all the terrible things that happened to us,” the sentence becomes grammar- and semantics-perfect.
We can generalize on this usage by saying that we can ensure that the expression “can’t help + verb + -ing” will always work properly in a sentence if it’s immediately followed by a complement, which can be any word or phrase that completes the sense of the verb. In the three sentences given as examples in the preceding paragraph, the complements are, of course, “all night,” “out loud that I’ve been robbed,” and “after all the terrible things that happened to us.”
