I think a similar argument would apply, that a reasonably informed user would understand that some metadata will be shared with (components of) the Tor network (I think this is a reasonable description of what controlling a guard node gains the FBI).

I understand why the FBI would need a warrant to capture traffic on some node owned by someone else. I don't see what would stop them from logging data from nodes that they placed on the network. Running a DOS is sketchier, but they do actually have some authority to not obey the law.

(please everybody try to separate the reality that they do have this authority from arguments about whether they should.)

edit: I fixed "Tor" in this comment and the one above, thanks to RemoteWorker.