My guess for the reason:

If you are a member you see the FTP passwd. If one gets your passwd (s)he gets the FTP passwd in the same step.
As it is not too easy to get membership the member passwd's have to be more secure to protect the FTP pass.
Particularly when there is a commonly used remember functionality, there should be something that can limit the (most times unintentional) leakage of member passwd's.