Guys, my only thought on the harsh restriction the US has imposed on Ukraine not being allowed to use US weapons to attack true Russian soil is the nuclear threat.
By this measure, Russia may consider an attack on its soil by Ukraine utilizing US weapons, as an indirect attack by the US against Mother Russia, thus justifying a Russian nuclear response against Ukraine. This communication may have already taken place between Russia and the US using back channels. If this is true, then it would explain why the US has been insisting on this restriction all along.