As to it being taken from him, this falls under another law called the CPL and it tells how to handle criminals and evidence ( parts of the PL cover this also). NYC and NYS have many, many laws. The summons could have been issued without taking the knife away, but the officer determined to his satisfaction that the knife was a gravity knife and therefore illegal so it had to be confiscated at that point. Had he not flicked it, or it did not open when he did, he would then not have had cause to keep the knife unless he could articulate that the person was not in the state of mind to have any knife at that moment( he was drunk, very upset, etc)