Considering that it would be an invasion of privacy and confidentiality for Atlassian to access the content of the board to assess which one is corporate and which one is personal, Atlassian to the safer approach to satisfy a paying client.
Consider that as a free user, with no advertising to monetize you, one could guess that Trello used you for advetising (Unless you are a paying user for your personal account, that could change the story).
Of course I am not big fan of the approach, because the user probably linked personnal and work account for convenience, and that trello probably didn't make it easy to make the switch between work and personal.
On the other hand, how do you prove that an email address is a personal one ?