Law is not some immutable force. Many countries have laws.
In some of those countries, Hamas is a designated terrorist organization. In others, it is not, and even considered and ally (or has been previously, such as Afghanistan, Algeria, Iran, Qatar, Syria).
Hamas its self is a government. They have their own laws. So whose laws should we defer to?
The point is that who is or isn’t a terrorist depends on the context and point of view you are speaking from.
There is no universality in that kind of word, and so its appropriate that the BBC isn’t using it.