You forget when Herc went and grabbed the portable mic from the spy store and stuck it in a tennis ball. Or when they stuck the video camera in the park and got it stolen.

It's pretty much the same thing that happened here. They knew the discussions were happening, they knew where the discussions were happening, and since it was in a public space they didn't need a warrant. It's not like they are running around town putting microphones in all the bus stops hoping to catch anybody saying something. They were looking for a specific type of criminal activity and they placed the mics accordingly.