When your wearing a vest that will stop a bullet (hopefully) but a knife can easily slice through, it tends to make you somewhat fearful of knives, if anyone can argue that point, please do.
But in the situations people are speaking of here I think its more or less when someone is stopped and they have a knife on their person, not that they have it exposed/open.
Are bad judgement calls about knives made? Yes, and people talk about them and they become stories passed down about some jerk cop that took my knive that I use for work or whatever.
Just like with many things in the NYPD and Police departments all over, the bad things get magnified and the good things get swept under the rug. A cop that runs into a burning building and saves 3 kids is a hero for a day. That same cop takes someones knive and he's a jerk now.
On that, ever wonder how many times a person is stopped and they do have a knife on their person and are honest about it, and the officer in fact lets them go with the knife? Thats probably not given much thought, cause no one passes those stories down or comes on the internet to post about them.
Its all the bad experiences that people talk about. And don't get me wrong, as I said, knifes are often taken when they should not be. But at least remember that there is a lot of times that they are not.