It could have been a friend that left them. However, I think its polite to ask the family members first (or at least the member that is the "care taker" of the grave, there always seems to be one).
My mother has the same sort of problem on my grandmothers grave. Except it is more of a tackyness issue. Her, my aunt, and I keep the grave tidy with real flowers planted. There is a lovely angel statuette in the middle. Since we are allowed to have real flowers in the ground with this cemetary, we spend a lot of time and effort, so much that it is a picture perfect garden over my grandma's resting place.
My grandmother was a considerate, gentle woman, but she did not care much for plastic flowers. However we have a stray relative that comes once a year or so to place plastic flowers and figurines on the grave. I know that it seems petty, and that we should be grateful for anybody who wishes to reflect on such a wonderful woman, but it irks those that were closest to her that they would just put whatever they want on a well manicured site without asking. The energy my mother puts into the grave is her closure, and perfecting it makes her feel closer to her mother. It just breaks my heart to see her silently fuming whenever we visit grandma to find another faded plastic rose with fallen plastic petals littering the graveyard. Its not so much that she detests plastic flowers, but the people who leave them rarely return, leaving them to blow around and fade.
So my rule of thumb is: Ask the close relatives first what is appropriate - they are upkeeping the site afterall.