1. They might not choose Healing as their primary focus.
2. They don't need to "report" to anyone that they did not choose Healing as their primary focus.
3. If anyone in a party NEEDs the divine that is already in the party to do a lot of healing, it should be on the person who needs it to make sure the divine can and is willing to do that. (Of course a divine shouldn't join a party requesting a healer if that's not something they want to focus on.)
I believe that people should build their characters in whatever way they want and is fun for them. And I don't believe people have to give a "disclaimer" every time they join a party just because their character might not perform whatever function someone thinks is attached to their class icon. Fortunately divines have other skills that contribute greatly to the party if they don't want to focus on healing. If you would like to build an arcane that does NOTHING but buff, maybe some people would invite you back. However even Bards who have been specifically wanted in parties to buff also contribute in other ways.