I think the confusion in this thread between theists and atheists stems from a misconception about why we leave decorations in the first place...
Grave maintenance and decoration don't do anything for those that have passed, for obvious reasons. But they do something for those that are still living, and serve as a reminder to passers by that someone who's alive right now once loved the people beneath the headstones. It's really no more complicated than that.
My children have no recollection of my grandparents because my grandparents all passed before my kids were born. It only takes one generation (maybe 2) before there will be no one alive who has a living memory of you. There will be nothing connecting you to your progeny other than the stories and habits they pass on in your remembrance.
Look at how many headstones are in disrepair and you'll see what I mean. Those vacant, overgrown, deteriorating areas tell a story about the person that once existed and about the family that forgot them.
The main confusion is that people claim to know how it all works, when they do not know.
After all this is just the same as with God, it is a belief.
You do not know, though you write as if you know it exactly.
So, although I also believe that the flowers don't do anything for those passed
I don't make a statement out of this. I could easily be wrong in this, as I have been wrong before in my life.
If I want to be sure I then have to die, when dead, I have to ask around "somewhere" how all this works
Then I have to get the "hell" out of there again, as a matter of speech. And come back to earth to tell you.
Something tells me this won't work out. Though it does sound interesting to do this IMHO
[Actually this is my experience. I once stopped eating and coming "up" were warned I was on a one way journey; then I was given a second chance]
[But even this could easily be concocted in my mind, because my bloodsugar when returning was 1 = almost coma]
Bottomline: In my humble opinion, I know nothing for sure. I even don't know how to cure cancer in this life. Let alone I know about the "after life".