Bulk mailers nowadays use so many tactics to make their stuff look important ("official" looking government-brown envelopes, "URGENT" markings, etc.) that a lot of it would probably get by a "no junk mail" order anyway. The post office can't open it to see what's in it, and if it's not 100% apparent that it's junk mail, they'd have to deliver it. They can't be making judgement calls. Ninety-nine out of a hundred pieces marked "URGENT" may be bullshit, but that other one piece could be very important.