All it takes is one person to then point out that if security had been good enough to catch actual terrorists, the suspects would have been caught before or during the act. Then not only is a chunk of the post-9/11 security apparatus exposed for the security theater it is, but the cops look lazy and stupid too.
The point is, you cannot prevent all terrorism on home soil through police enforcement. You can choke certain points like airports by doing person-by-person inspections, but you can't do much for land-based transportation infrastructure with millions of autonomous attack vectors. Terrorism is best prevented through political actions.
The majority of people are probably indifferent to this. But for some reason those people who wouldn't mind NYPD not wasting money on this investigation are less vocal or less influential or something compared to the pro-security-state advocates whose position is "OMG could have been terrorists!" The latter may be mostly bureaucrats in power (perhaps the reason for the overriding influence of this paradigm) worried about it for job security reasons, but the motive doesn't matter.
This simple act has embarrassed an entire industry, including countless highly-paid public officials; the very same people who determine how much resource is dedicated to soothing their bruised egos.
