Why is sniffing a flower considered to be stealing?
I guess one possibility is that "steal" is an inaccurate translation, and that a better translation might centre more on the "conceal" and "stealth" meanings of the words (e.g. concealing or taking a scent by stealth, secretly ... and observed only by a deva).

Another possibility is that it's taking what is not given: what is given (by lay-people to bhikkhus) are "requisites" -- food, medicine. The scent of a flower is not a requisite; taking the scent might be considered a sensual craving.