I agree that it sounds a bit harsh but people need to know that there are rules on what can and can't be left on graves.
Also, if someone is going to put stuff on a grave, they should have the courtesy to come back and clean up after themselves and not leave the mess for someone else to clean up.
Imagine how long it would take to power wash the thousands of markers and then imagine having to stop and clean up messy, albeit well intentioned, trash.