لِأَنَّهُ الْمُتَبَادَرُ إِلَى الْفَهْمِ
Because that meaning is understood immediately.
Tabādur is the immediately apprehended sense of a form in the absence of a special clue. Normally lā tafʿal first suggests a genuine prohibition, supporting restraint as its primary signification.
A primary meaning does not exhaust contextual usage. Speaker–addressee relations, tone and circumstances may lead the same form to signify a threat, supplication or entreaty. Those shifts are indicated by contextual evidence.