Like it was stated before, anyone can buy the nalia, they may be giving it as a gift. If you are wearing an organizations letters however, it means you are a member of that organization. There are ways (handshakes, passwords) that members can tell if you are really a member. The few cases I have seen where someone was bold enough to pretend to have earned their letters have resulted in a bad beat down and a stripping of the nalia that they did not deserve to wear.
If you want to be down, join, because as most organizations say "Membership has its privileges"