The flowers basically were 'heaven sent' in the old days, probably still are somewhere, where an angel basically trod on holy ground they'd spring up.
As a 'fuck you you bastards' to the demons though, god decided to have the earth react to the demons summoning (which takes a lot of magic and energy anyway) by sprouting these bastardised versions of the flower as a subtle reminder they let him down/failed/ ain't going home any time soon. I'm thinking they may also be mildly poisonous where as the originals were good to boost physical health and help cure illness which is considered an old legend rather than fact now. 
What do you think?