I'm not a lawyer either. I was wondering about this a bit and tried to see if any of the restaurants where Mafia members met in the 70's took any heat because of crimes that had been planned within the establishment and could not find any. (it was the only analogy I could think of which the FBI might use as an example, if there are better ones please add them).
That being said, I would not be surprised if the FBI freely used a 'National Security Letter' [1] to force these guys to turn over data about specific connections. And since, as a service, these guys may be subject to the requirement of giving the FBI a way to wiretap their connections [2] it may be that once they knew the service was being used it was a 'simple' matter of compelling them to provide an unencrypted copy of sessions of interest.
That being said, I would not be surprised if the FBI freely used a 'National Security Letter' [1] to force these guys to turn over data about specific connections. And since, as a service, these guys may be subject to the requirement of giving the FBI a way to wiretap their connections [2] it may be that once they knew the service was being used it was a 'simple' matter of compelling them to provide an unencrypted copy of sessions of interest.
[1] http://en.wikipedia.org/wiki/National_Security_Letter
[2] http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-05-153...