I think this ghost bike thing has been a colossal mistake, helping to make bicycling look more dangerous than it is. I can't think of another instance where memorials are commonly another of the type of vehicle the person was using when they were killed.
Frankly, if there was one of these things in front of my home 6 years after the accident and the city was too cowardly to remove it, I am pretty sure I'd cut it down. Life needs to go on on that street, it is not going to forever be known primarily as the place where a bicyclist was killed.
Everyone has, if they've lived long enough, experienced the tragic loss of a loved one. Like it or not, we have no right to a permanent symbol of that grief on public property. And, really, if everyone was entitled to such a permanent monument at the place they were killed, the cities would be choked with them within a few decades.