How do you do that? Once she joins the team, you have no way of knowing if something Ann created or modified belongs to the team or to her. This is the classic problem with "taint checking." Once you mix data across boundaries you have to assume it's tainted. I agree it's a crappy situation, but I can't see what Dropbox should change, other than make it very clear how this feature works to future customers. As a one-time thing, they could maybe get a PR win by helping this guy recover his data (if possible). However, that's not scalable, or a precedent they'd want to continue forward.
They can clearly delineate between team and personal accounts, force team storage to certain directories, and/or not allow users to manage team/personal accounts with the same login.
I was specifically responding to the previous comment, to explain why Dropbox can't simply revoke access to team storage files at this point (because they can't guarantee what constitutes team data). However, I'm in agreement with you that Dropbox could introduce new features to prevent this situation from happening in the first place, and had already noted as much in a different comment.
Dropbox doesn't delete the files, so none of this matters. They're only deleting the account, which revokes nothing but causes lots of problems. There is nothing hard to fix. Just switch it back to personal quota.
