Terrorist is a vague word, bomber is a specific word, and therefore more relevant in a news report.

News reports ought to be about providing the facts, not about telling people what they should make of them. There is a place for comment, but there should be a distinction between comment and factual reporting. "Terrorist" is a word that blurs that distinction, and is often used for that purpose. It is better kept on the comment side rather than the fact side, and I would imagine that the BBC has something like that written into its house style manual.