There's a difference between obstructing (generally bad!) and refusal to deal (perfectly fine). Apple can't go out into the market and trip their competitors so they fall on their face. However, they are allowed to build themselves a nice road for their exclusive use on their own hardware and a winding dirt path for everyone else.

Put another way, Apple cannot worsen access for competitors on its platform absent a procompetitive justification. But they have no duty under U.S. antitrust law to provide equal access in the first place.