The police had to act on them, and seeing the pictures of them taking down the devices it looks like once they were on scene, they got the idea.
However, they had to get all the devices, because it would be a clever thing to put up 38 devices, including a few in harder-to-find places with real explosives, hoping that once the police found a few and realised they were harmless they would let down their guard, and you could remote-detonate the real ones when you had enough police officers gawking over them.
Yep...like I said, better safe than sorry. It's a sick world we live in and unfortunately too many people simply do not want to see it.