Firstly, my condolences to all who were killed or injured and their relatives.
As for garbage cans at a location where literally thousands will congregate, that is a no no. Years ago we were in Waterloo Station London and had enjoyed some snack foods while waiting for our train. I went to dispose of our garbage and could not find a garbage can. I asked a Bobby why there were no garbage cans to be found and was told "bombs you know". Mind you the Brits have been at this counter terrorist bombing routine quite a bit longer than we have in North America and probably learned the hard way too.
Bob