I'll not argue your point, but I think the IMO does illustrate why the BBC is absolutely right to avoid using the term. Various governments, organisations, people try to define terrorism, but always meet the response "well, didn't the USA do that in Vietnam" or "didn't the British do that in Ireland", and they did. So now it is generally left undefined, or defined by diktat (see "it is a fact of law" mentioned above). In other words it is a yah-boo word, it means "use violence that I disapprove of", it has no place in reporting.