The difference between terminally ill and 'brain death' people is interaction. Terminally ill people can still act like a person. They can express their feelings to you, they can act on their feelings etc. This contrasts with 'brain dead' people. Not only is their no hope for recovery, there is also no current interaction.

There are obvious questions raised in the above line of questioning about grey areas. For example, people who are terminally ill and mute and bed-ridden. Or indeed the girl from the article who was 'brain dead' and yet seemed to have some way of expressing herself.

My point is that there is a basis for distinguishing between terminally ill and brain dead people. Though that raises hard questions regarding the gray area.