I think they are doing the aplicants a huge favor. They have open interviw slots because people probably canceled on them, and rather than just let the spots die, they try to give people who want to go to Northwestern a good chance to go. I don't think it is messed up in the slightest. I'm sure there is a lot of people who would love to pay 300$ more on a plane flight and have the interview than not have it at all.