This article suggests that
Clostridium difficile spores (mentioned in TFA) can be found "on hospital items such as over-bed tables, side curtains, lab coats, scrubs,
plants and cut flowers, computer keyboards (especially computers on wheels), bedpans, furniture, toilet seats, linens, telephones, stethoscopes, jewelry, diaper pails, and under fingernails."
As flowers can not be cleaned and disinfected as easily as the other surfaces mentioned, this seems like a reasonable preventative measure to me.