The problem you have here is that the beginner you are training with cannot take Ukemi yet. While training with a a beginner you are only running slowly through the technique to get the movement right. You can apply the technique with power and speed on higher grades who can Ukemi at a high level of skill. The reason it seems as though Uke is throwing him/herself is that Ukemi is our way of avoiding injury. On the street the person just goes straight into the ground. All this is, is a case of adjusting your technique to the level of your Uke.