I see your point - haven't been there (luckily!) at the moments of attacks, so I probably didn't see the full police operation that followed. My overall impression is that NYC is a city flooded by police and security at any time, not that it is specifically preparing for some thouroghly planned attack.
There are many things going on here, and especially as the elections are approaching, the security forces are multiplied, I think this is very expected.

But again, this is just my personal and uninformed opinion)