U.S. and British law dictionaries are written for U.S. and British lawyers. Lawyers trained and working in the common law system. Lawyers whose native language is English. They don't use an English law dictionary in the same way as a non-native speaker trained and working in a non-common law system.
A British or U.S. lawyer looking up the word 'decree' might want to know the difference between a 'decree in equity' and a 'judgment at common law' (something relevant only to the common law system). Lawyers who are non-native speakers of English will most often want to learn other things like how to use the word in a sentence, how to pronounce the word, or what common mistakes to avoid. They need a learner's dictionary of law.
That's why TransLegal has put its more than 20 years of experience in producing Legal English tests, course books and online training into the world's first learner's dictionary of law.