The gov website article on inheritance tax states that gifts to charities and political parties are exempt from inheritance tax. Why do political parties get this special treatment?

We think there is no good reason gifts to political parties should be exempt from IHT. In some circumstances people who have received gifts have to pay inheritance tax and we don't see why political parties should be any different. When there are what we think are outrageous rates of IHT on other gifts (that has already been taxed multiple times in our relatives' lives); what makes it fair that this doesn't have to apply to political parties? We think the politicians seem to do whatever they like.