So I have a slightly different and more sinister take on Penn’s new policy. I think it is attempting to help Penn’s RD yield. Right now, if an ED applicant is deferred, the student might get into Chicago, MIT etc. Those schools get 3 months to convince the student to attend and students will probably accept their offers during that period, either because they received a good financial package, or because they become attached to the school. If those deferred ED applicants are forced to apply to Chicago etc. in the RD pool, it is less likely they will be accepted. Thus, if they ultimately get accepted to Penn in the RD round, they will be more likely to attend Penn - I don’t know how many students fall into this category (deferred ED who then get accepted RD), but I suspect it’s enough of a percentage that it might affect the ultimate yield.
Maybe I’m just a conspiracy theorist, but I can’t see any other reason for this policy.