I think organizations are intended for a small group of internal users and that’s the reason why it’s limited to internal and whitelisted users. I think the goal is to restrict the amount of users since the pricing is designed to bill per user per month. If you opened it up to public with email, then you open yourself up to the potential of hundreds of users getting a hold of the link and using the app. You would probably be billed for all of those users, even if it wasn’t your intention.
If you are only working with a small number of users, then you simply need to get their email address’s and place them in a sheet that the whitelist option will point to. To the user, it will work just like any other public with email app. They just need to have a their email in your list.