Dropbox deleting the local copies are effectively what the author wanted to happen. The author wanted Dropbox to:

1) Remove the local copies.

2) Keep the remote copies.

3) Stop syncing those folders.

The author explicitly states that the goal was to use the Dropbox copy as the only copy (i.e. the backup).

Oops, you're right. But the way I read the report, even having other systems with local copies would've gotten data loss, because it would have removed those local copies as well (which was not the idea when enabling selective sync only on the laptop).