The reason colleges don’t rescind for a few Bs is that, by the time they find out about spring semester grades, it’s far too late for a kid to apply anywhere else. Especially in the case of an ED college, where they have told the student to withdraw all other applications. Withdrawing an admission offer under those circumstances is pretty harsh. Granted, I know they do conditional offers in England and elsewhere, but that’s clear upfront, and it isn’t what happens here.