They are needed in the ADIZ on the way in and the way out but there is no one to enforce the requirement on the way out.
45.29
(h) After March 7, 1988, each operator of an aircraft penetrating an ADIZ or DEWIZ shall display on that aircraft temporary or permanent nationality and registration marks at least 12 inches high.
45.22
(c) No person may operate an aircraft under paragraph (a) or (b) of this section--
(1) In an
ADIZ or DEWIZ described in Part 99 of this chapter unless it temporarily bears marks in accordance with Secs. 45.21 and 45.23 through 45.33;