The FBI has copies of the servers TorMail ran on that they legitimately seized in an unrelated investigation (the servers were also hosting child pornography websites).
In the course of another investigation, the FBI found that orders for forged credit cards were being sent to a TorMail account.
The FBI obtained a search warrant for that specific account and then accessed it from their own copy.
This is not trolling the seized database for anything and everything that might be illegal. This is finding probable cause from another source and obtaining a specific search warrant. This is how it is supposed to be done. Why would you expect anything less from competent law enforcement?
The FBI is not the NSA. FBI cases have to hold up in the light of open court.
If you are upset about the fact that TorMail was not in fact secure, well, that's on the TorMail operators and on the users for trusting the entity that controlled TorMail while knowing absolutely nothing about them. Remember, TorMail has nothing to do with the Tor protocol, and is just the name someone gave their supposedly secure and anonymous e-mail service that they hosted on the deep web. For all anyone knew, the FBI could have been running TorMail all along.