That is, unless the server simply stores your password in plaintext, in which case it can email the password back to you when requested. This is why you would normally get a password reset email instead.
This forum isn't doing that, it only sends you your password right after you signed up, because it already has it in plain text at that time. There is no way to retrieve your password as plain text again because it is stored as a hashed value.
I don't know how you managed to reach the conclusion that just because it sent you your password once, it stores the passwords in plain text and can send it to you again. Have you seen a button that sends you your current password in an email? I don't think so.