El,
Assuming for a moment that your aircraft is G-reg passenger transport, then you are not permitted to fly instrument approaches in controlled airspace using kit that is not FM immune. Off the top of my head, I can think of relatively few reasons for your placard.
a) The placard is out of date
b) The placard is warning you that ILS signals are no longer protected from FM broadcast stations
c) errr...
d) Your twin-FMS guidance is regarded as being sufficiently accurate to replace the localiser as the primary source of LNAV data.
Scraping the bottom of the barrel here.