In addition, terminal B has three separate wings, each with its own security checkpoint and no way to get between them. United's Star Alliance partners (and I think very rarely United's own flights) depart from the northern two piers of B, mostly the northernmost one, while all of Delta's flights stop at the southernmost pier of B (which is used exclusively by Delta), so it would be very unlikely United even has a place to stop the bus at the Delta pier.