Learn Italian: Mi dispiace che nessuno abbia aiutato la vittima della truffa. - I am sorry that nobody helped the victim of the scam.

The phrase Mi dispiace che expresses an emotion. In Italian, verbs of emotion, doubt, or personal opinion require the subjunctive in the following che clause. We use the past subjunctive (congiuntivo passato) because the lack of help happened in the past.

Why isn't there a non before the verb, since Italian usually uses double negatives?

When a negative word like nessuno (nobody) is placed before the verb, you do not use non. You only use the double negative if the negative word comes after the verb, which would look like this: ...che non abbia aiutato nessuno.

Since the lesson is about the pleonastic non, is this an example of that grammar rule?

No. The pleonastic non is an "extra" negative word used in certain comparative clauses (like più di quanto non pensassi) or after verbs of fearing. This sentence simply follows standard negation rules for the pronoun nessuno.

Why does the sentence start with Mi instead of Io?

The verb dispiacere works structurally just like piacere. It literally means "it is displeasing to me." The mi is an indirect object pronoun ("to me"), and the entire following clause acts as the subject that is causing the sadness.