wiretapping normally require a specific target, with a specific reason. Going after the tor email service, is like wiretapping the US postal service for a fishing expedition.
It sounds to me as being outside the FBI's legal wiretapping abilities.
First, you are implying that Tor has an official Tor e-mail service, which it does not. Tormail is/was just a basic e-mail service someone not associated with the Tor project was hosting on the deep web. For all anyone knows, Tormail itself could have been run by the FBI or NSA or whatever all along. Anyone who thought Tormail guaranteed them anonymity was a fool, much like anyone who kept Javascript enabled while browsing the deep web was a fool.
Second, Tormail wasn't itself targeted. What was targeted was the hosting provider that was hosting 95% of child pornography in the deep web, and that hosting provider also happened to host Tormail and a bunch of other non child pornography websites.
Conspiracy theories will abound, of course, but keep in mind that the NSA's MO is not to disrupt communication but to intercept it. If the government's real concern here was with Tormail, they would have simply kept it around and tapped it, since they had clearly compromised the hosting provider's boxes and could have done so. They wouldn't have shut it down and just sent people fleeing to the dozens of other supposedly anonymous and secure e-mail services out there, including ones that perhaps they haven't yet compromised.
> What was targeted was the hosting provider that was hosting 95% of child pornography in the deep web, and that hosting provider also happened to host Tormail and a bunch of other non child pornography websites.
What the government did was the equivalent of show up at the houses of everyone who used a particular post office and forcibly finger print them because that post office routed 95% of the child porn magazines in the US (regardless of what percentage of their traffic that actually was, which you don't even mention besides 'there were other sites, too').
That would be a clear abuse of powers, as is this.