They are not asking Apple to give iMessage to Samsung. If WhatsApp wants to have interoperability with iMessage, then Apple should enable the interoperability without compromising their security. This is done by exposing some suitable APIs. The result of this interoperability is that, if an iPhone user wants to shift to Android, the user should not be held hostage because all his messages are in iMessages. The user, through the APIs, will be able to move them to the messaging client of his choice. Then, the only reason a user will stay with an iPhone is because it is good, not because Apple held the user hostage.