This is not true. The Australian Crime Commission has some of the most extensive secret coercive powers in the Western world.
http://www.austlii.edu.au/au/legis/cth/consol_act/acca200228...
I would suggest that either:
a) Fastmail is aware of this and is covertly spreading the word that it might be compromised; or
b) Fastmail needs better lawyers.