I think the idea is that air space provides a thermal break, even with an insulated ss liner. The other concern, of course, is settling or other uneven contact between the liner and the insulating material. I have loose perlite around my liner, but for what it's worth, from p. 21-22:
"Just as with clay flue lining, stainless steel liners
require space between the liner and the
surrounding masonry wall to limit temperatures
on the chimney exterior. However, in order to
pass the stringent limitations of UL 1777, most
stainless systems must use some form of solid
high-temperature insulating material rather than
an air space. When they were first introduced,
many stainless liners were insulated with a loose
fill of vermiculite or similar material. This is no
longer allowed because of concern for settling and
sifting of the material into the flue. Listed
stainless liners must provide a specified amount of
space around the liner to allow the proper
thickness of insulation."
http://www.csia.org/Portals/0/CSIA_ChimneyFires_Causes_Effects_&_Evaluation.pdf