When you say "clearly stated" do you mean a guy at a dinner said one year? If so, that's generally the case. DL is somewhat unique with the 400 hour gateway, also a negotiated benefit. Most probationary pilots are apprentice members for exactly one year to coincide with their probationary period. Once off probation, dues start; that is common throughout the industry. When you are no longer on probation, you can vote and have access to the system board, etc which is why you pay dues in the first place.

AFAIK they have been taking dues at the end of probation for a while. Its great that you called and asked, but its absolutely fair to charge dues once off probation so if that's the way it is so be it. Its really nothing to get wrapped around the spray deflector about. Its common practice and many thousands of pilots before you did the exact same thing. No dues coincides with probation. Dues coincide with non probationary pilots. That's the way it is pretty much everywhere.