Yes, the FFL might not care whether or not Sg govt allow, and
ultimately you can just ignore the sg govt. But the issue of
whether or not you are allowed to join is not irrelevent for a
number of obvious reasons (eg Coming back to visit relatives,
Parent's funeral?, etc.)
And ya, i Fatum, there is possibly some kind of law against joining
but then there is a possibly there isn't.
Anyway, this topic has been done before. Do a search on a thread
along the lines of "Joining the French Foreign Legion", started by
yours truly.