People have a right to give up their Constitutional rights.
You can give up your Fourth Amendment rights and allow the police to search your house without a warrant.
You can give up your right to counsel.
Etc, etc...
Yes we can give up our rights but what should be the standard of evidence to determine that we did, especially for the incapacitated or incompetent? On face value the Florida law doesn't seem that out of line since it calls for clear and convincing evidence (though I suppose some may argue that beyond a reasonable doubt would be a better standard). But Greer seems to have taken great latitude with the meaning of clear and convincing.