The basis of this is that someone is being taken off a service since they are too unwell to travel. Given the timelines, this is allowed without confirmation, and yes I guess there is scope for gaming the system. However that's not entirely why a clinical letter (it doesn't have to be from a doctor) is required: it is also to indicate that the traveller is now fit for travel, rather than perhaps trying to travel before fully recovered. GCHs are generally waived this requirement for paperwork.