Yes, that is a very good point and while in real life "not allowed" would be best there is the translation problem when trying to teach because "not allowed" would then entail using a negative phrase. That can cause confusion. It's a fine line we tread. "Proscribed" was another idea but again sounded too strong and not very common. Thank you again for a very valid observation.
Then again, that certainly doesn't mean that it's the most appropriate word to use, just because it's a translation. Especially if it's just demonstrated so by a dictionary.
Dictionaries include all translations of a word,no matter the actual context of the sentence.