Why would news reporters trust bin laden not to kill them?

Bin Laden's ego. If bin Laden killed those reporters, then they couldn't get his message out, either positively or negatively, and grow the cult of personality around bin Laden. Bin Laden's success comes from his political savvy and ability to manipulate public opinion. Contrary to conservative spin, he was not so ruthless as to want to murder every westerner he could get his hands on.