I dunno, fixing the market to be “company X’s own services” doesn’t seem to be in the spirit of antitrust laws. Should I be allowed to sell gasoline at Shell’s gas stations?
You should be able to sell your gasoline to customers with Ford's cars, regardless of whether or not Ford has their own gas stations.
But why are we reaching for a car analogy? Should gmail or google.com be able to block Firefox and force you to use Chrome? Not make use of some feature Chrome has, but just purposely block Firefox even if it supports that feature or its users are content to use the service without it.
