Hi Tom! Thank you for flagging this for us. To ensure that I’m understanding the situation in full, are you referring to the Use this data popup that offers drop-downs of available bases to create within? If so, rest assured that this is utilizing only the workspaces/bases in which you hold Creator-level permissions or above, and if users are signed out or don’t have an Airtable account already, clicking Use this data will lead them to a sign up/sign in landing page (where they could then proceed through to see their own list of available bases).
For further details on securing view share links, please feel free to browse our support documentation on the topic here. If I'm misunderstanding at all, or if this continues to exhibit unexpected behavior, please let me know and I'd be glad to dig further into this for you.