Terminally ill patients, or those in a permanent vegetative state, can take up valuable hospital beds for those who do want to get better. If they do not want to live, then they should not be allowed to take the beds and care of those that do.
Long term palliative care for the terminally ill is a huge and ultimately wasteful drain on medical resources. Why waste these precious resources on someone who has expressed a desire to die, when they could be improving the life of someone who wants to live?
In addition, these resources could be re-allocated to further the research of the specific disease the patient is suffering in order to allow future generations to either not have the disease or increase the quality level of care for future patients of this disease by alleviating the symptoms of the disease at the very least.
In addition, if the patient is an organ donator and the organs are healthy, it may save up several lives which are ultimately invaluable.