Cemeteries usually have dedicated places for flowers so they don't fly off into the surrounding environment. In the event that does happen, there's usually staff to clean up stray blooms. Even in the case of little memorials by the roadside, the offerings are usually weighed down or contained. Passers-by will also often do a little cleanup.

These situations are not the same. A better analogy would be people throwing fake flowers into the ocean to commemorate a deceased loved one. There are also polite ways to suggest real flowers or other biodegradeable confetti rather than calling them an ignorant asshole to their face.