It's not. It's either an inconsistency, or a bug.
alias blah { $iif(0,halt) ; test }
This gives me "; test" and the "; Unknown command" error.
alias blah { $iif(1,halt) ; test }
This gives me "halt ; test" and *that's* why "; test" is ignored there, because it's treated only as parameters for /halt in the first place.
