I don't consider people who go on tons of interviews greedy by any stretch of the imagination. Logically, if you have the means, it makes perfect sense to attend any interview where you could realistically see yourself attending.
However, I do consider it unconscionable to attend an interview if you know you will not attend. I know two people who made a bet against each other two see which one could get more medical school acceptances, and one of them is going to attend a March interview at Rosalind Franklin after receiving an acceptance from UIC, Northwestern, and Michigan State. This is not a game. You're not trying to rack up points. By attending an interview at a medical school where you will not go, you are willfully taking away somebody else's chance at that school. How somebody could do that is beyond me.