Unfair Insurance charges

If there is one thing that NEVER, but NEVER ceases to amaze me it is how Insurance Companies operate. My son, busy with his learners, drove in Johannesburg and took a slipway to the left onto another road. He had to stop because of oncoming traffic. The next minute a young man in a expensive car comes down the same slipway and rams into him. The damages to my daughters car he was driving is extensive but the other car hardly. At the end of the day WE have to pay an excess fee of over R6000.00, reason being, my son is under 20 years of age or something like that....... WHAT? he did not cause the accident !! Why are we liable for the excess? As far as I know the insurance company has to recover the money from the pesonn who caused the accident....do they claim less the excess? or do they claim the full amount as invoiced by the repair company ? This is the worst *****ery I have seen.