There are two main reasons for this charge. One is that hospitals used to suffer from people meeting there, travelling into town in one car and leaving the other cars on the hospital car park. So they shared one parking charge between them. The other reason is that tax rises are bad publicity for the government. Much better to allow parking charges instead. The hospital will allow you to park on their (our) land subject to conditions. Among those conditions is the parking fee, which they can recover as a civil debt if it is unpaid.