They largely are. It's incredibly difficult to stop crimes before they're committed, and prior to any terrorist mania it was generally considered to be outside the scope of police work. Mainly because it requires a security apparatus, and authority, that is in direct conflict with an open democratic society.
This is the nerf-ball version of challenge to government authority, so instead of Tahrir you get a few kids changing a flag, and instead of a repressive government crackdown, you get increased police investigation. But the mechanisms are the same.
States, whether they be national or local governments, don't react well to open challenges to their authority, and they use every tool at their disposal to prosecute anyone that commits a crime that openly challenges the authority of the government.
Listen folks, you can't stop terrorism. A determined attacker will just find an easier target. You can't guard everything all the time.
No, but you can put everyone under the constant threat of imprisonment for nebulous reasons, or no reason at all, and there is some glitch in human reasoning that conflates the two.
100,000s of people do that every single day. It is a bridge. That is what bridges do. They carry largely unknown people across a highly visible structure.
I think you are blowing this out of proportion. Or maybe I shouldn't use the word blowing.
It's about the symbolism, not the act. If NYC, for all its counterterrorism efforts, can't catch these pranksters, it makes the public safety officials look impotent in the face of real threats.