Changing cause of damage on their terms

PWC51070H (***2) - T442876

The mechanic reported the cause of the damage was a stuck thermostat. MotoRite are now saying the cause of the damage was over heating so they will not be repairing the damage and not even covering the labour of the repair work to the engine because of the stuck thermostat. This is a joke beyond, how can you change the cause to something that suits you and ignore what the workshop who needs to do the work says. Of a R24000 quote you will cover R2000????