It appears that when you create ideas from within Aha on the Ideas tab, and not within the Ideas Portal, you cannot default those ideas to be visible to all Ideas Portal users. We go in and update every Idea to be public, so it would save us a bunch of time if you could configure that at the Product or Account level!
This feature is purposely designed in this manner. When initially creating this feature, we received community feedback that a large majority of users would like the ability for internal ideas to "stay" internal unless indicated otherwise.
This feature is designed in such a way so that these ideas can initially be private, but can easily be changed to be made public -- as opposed to the other way around which limits the ability to keep any ideas internal to the team.
At this time, we do not have plans to make changes in this area because of the rationale noted above, along with current priorities.