I don’t know why they do it, one would think that a simple deferral for at least qualified applicants would be sufficient. Perhaps it reduces the volume of non-serious candidates? Or minimizes the applications from kids who may be trying to “game” the ED system and may not really have a school as the absolute top choice, but are just trying to get in to the best possible school by committing through ED? Oberlin did say they do a financial aid pre-read for students considering ED so that they know the financial parameters before committing. I could imagine that process – of possible rejection and financial pre-reads – means the ED students you get are really going to attend.
Living in Indiana, we know many applicants to ND, and they are routinely advised not to apply EA to ND unless they are at the very tippy top of applicants, because the risk of rejection is real. Again, maybe it is to shrink the EA pool to students who are very serious about attending and could be admitted to top 10 schools, so that an EA acceptance increases the chances that student would be primed to accept ND, even if there are some spring acceptances to higher ranked schools?