It took a bit of digging to find this :
Awake 1977 3/22 p15
Another personal feeling affecting reactions to funeral customs is the desire to keep a funeral simple, free from ritual.
This could involve, for instance, some customs followed at the grave site. In certain areas the pallbearers or family members are expected to put a flower on the casket or to toss a flower into the grave before it is filled. To many persons this is considered a final token of respect or a last tribute to the dead person. But, of course, the true Christian knows that the deceased is not aware of the flower. And, if the deceased had been a true Christian, he too would have agreed with the counsel at Romans 1:25 against giving undue or worshipful honor to a creature. Hence, the personal feelings of some individuals have led them to omit this custom
I have realised that your location , Bonsai, and that probably accounts for the strict application of the rule.Even so, it seems an extremely tough stance to take. Here in the U K I have attended Witness funerals where flowers were on the coffin , and nobody gave it a second thought.
At least it helped you to see the truth about The truth so maybe a blessing in disguise.. Welcome to the board.