In some countries, the concept of punctuality and time management doesn't exist. I remember the missionaries telling us that in many African countries, people wander into a church service whenever they get there--there is no concept of "starts at 10:00." And they stay as long as they wish--there is no concept of "ends at 11:00."
So punctuality and time management are only virtues in certain locations (like the United States), and you probably shouldn't get so emotionally-vested in this issue. In other countries, you would be out-of-line.
I agree with those who say that it isn't your job to teach anyone anything, unless you are a teacher or a parent trying to teach your own children.
I don't think you have any right to inform anyone that they are wasting their time or frittering time away, unless, of course, you are the "boss" or supervisor or C.O.
My personal feeling is that in the U.S., we are too aware of time and we habitually overschedule our lives so that we won't "waste any time." I think we all need to step back and spend more time just "resting" and "being" and "praying." Being over-worked and over-committed is not healthy at all.
I do think it's reasonable, since we live in the U.S., that if you make an appointment with someone and they don't show up on time, leave, or close the door (if you're in an office). You don't have to be snarky or crabby about this--just inform the late person that you couldn't wait for them because of your other responsibilities.