This one hits a bit close to home for me. I'm actually just finishing up my PhD in the School of Computer Science at Carnegie Mellon. Within a month, I should be Doctor Atog. Getting an acceptance letter like that can be life-changing. I've spent the past six years of my life in Pittsburgh because of being accepted to CMU. This has been an amazing place and I feel very fortunate for the opportunity to have been here. I've had doors opened because of being here, and I've been able to have some very rewarding experiences. I've learned a lot and I've certainly grown as a person. I still remember first getting that acceptance from CMU. I was overjoyed, and I knew that my future would be different because of that acceptance.
The students getting these false acceptance letters had several hours before there was a correction. Those hours are a lot of time. That is enough time to tell present employers that they are quitting. Enough to tell friends and family the good news. Time enough to tell other schools that they are retracting their applications. In other words, lots of time to make some fairly hefty life-altering decisions based on the news.