When migrating an account into a team, the account and all the data in the account becomes managed by the Teams admin (shared or otherwise). Letting an existing account to join a team lets us smoothly support the situation where a user has created a Dropbox account separately and needs to move that account into the team.
For most cases though, users should create a new account for the team. The Dropbox for Teams sign up process guides users towards creating a new account when joining a team for this reason.
From your reply, it sounds much worse than I thought. I had no idea my team admin could see all my data until now!
If joining a team account changed after your October changes, then maybe it's okay now. But I included in my post the email I got for joining the team, and it makes no mention of the fact that the team admin now owns my data. One would think this email should be less bland and more cautioning.
For most cases though, users should create a new account for the team. The Dropbox for Teams sign up process guides users towards creating a new account when joining a team for this reason.