I was surprised to see BS offering ED as well and agree that it creates a pressure that may not be helpful.
With that said, in many cases where the Upper School is a BS and is part of a K-12 school, the students who are coming from the Lower School have had different and earlier application and (binding) decision dates if they want to continue on to the Upper School. At some schools, that ED option may be more equitable for new students. (Wondering if Westtown is in this group.)
Day schools are on an earlier cycle as well (often forcing students to deposit at their favorite day school before they get a BS decision.) Many offer an even earlier decision date (and priority) for 8th graders who want to continue to 9th.
This is to say that this is a surprising trend, but perhaps not completely new.