Perhaps the person who wrote to you is unaware of the Ghost Bike protocol, and thought you were leaving it up forever instead of just for a week. Maybe they thought you were going to abandon it, rather than take care of it and clean up afterwards.
Would it help these folks on Freecycle if you explain the entire concept and how you will be responsible for it in your ad? They might just have no clue about the Ghost Bikes.
Once you get the bike up, it would be very cool if the local paper (is that the one with all the articles you linked?) came out to take a picture of the Ghost Bike near the start of the memorial week. I would imagine that other local folks would like to pay their respects before the week is out, too.
It sounds like it was a brutal accident, and that's traumatizing for everyone.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson