I wondered that as well. It seems that in NYC, pre-RNC, the rides were an accepted happening. The mood jovial, the police friendly. According to friends there, it changed when the big boy security (FBI, Secret Service) came to town pre-RNC. The NYC police suddenly got serious.

The organization that (I think) started the rides in NYC is Times Up! http://www.times-up.org . That's all I know about it.

It was quite an experience... I guess the response could be different in each location.