Your grandfather declined life support because he was dying. Terri was denied food and water because she wasn't dying.
Michael Schiavo used to complain, "When is that b*tch gonna die?" It took him many more years and all of HER rehab money to put her to death, but y'know, she was, like, in the way.
Terri's body was being artificially fed through a surgically implanted feeding tube because she was unable to voluntarily swallow enough nourishment to keep her body alive.
This artificial life support was withdrawn, and her body was allowed to die a natural death, because it was her expressed wish that she not be kept alive this way.
Terri wasn't denied food and water because she wasn't dying -- she died in 1990.