These are my reasons:
There is no such thing as "organic food safe flowers". Yes, you can buy organic roses, but that does not mean they are food safe. That just means they were grown with organic compost and no chemical pesticides. But think about this> Poop is 100% organic! Anyway, the fields are not treated with the same sanitation standards as they are if you are growing food (i.e. porta potties available to field workers), they are not picked, packed, or shipped with hygiene in mind at all, and flowers are regularly imported from all over the world where standards and conditions are very, very different then the US.
I have yet to have a florist hand me flowers for the cake that have been cleaned or treated like something that is being put in food.
In the case of your cake, yes it's extremely beautiful, but who did those flower sprays, the cake maker or the florist? If the florist did, how much extra do you think she charged to make those? Looking at that photo, I think yes - those look expertly arranged and I'd venture to guess the florist was paid several hundred dollars to make those. But if the if the cake maker did, how long do you think that took on site? Do you think they were paid for that time and supplies? Probably not because it's become industry standard for florists to sell cheap untreated flowers to brides to have the cake maker stick on the cake, otherwise this trend would have never started. If the cost of cleaning and arranging was included in the retail price of the flowers, it would no longer be a cheap alternative. I don't know about everyone else, but I have yet to deal with a bride willing to pay for the extra arrangements with a florist, they expect me to just do it for free.
I like making sugar flowers and it's a small way I can recoupe my time decorating the cake.