The way it works is that of the revenue identified as going to the players, out of that revenue the NRL pays a flat fee to the RLPA which is how all NRL players are covered by it without any money coming out of their current payments.
Problems this could lead to:
1. If a player objected to being unionisedc by default. It is illegal to make people compulsorily join; and
2. The NRL could just decide to stop the payments. Make the RLPA collect its own membership fees from players.
Looks like no. 2 is being conmmsidered by the NRL according to the Herald -
The RLPA receives $3 million from the NRL each year, but that funding is under scrutiny as the long-running CBA dispute between the organisations continues.
www.smh.com.au
Whilst it would inflame matters, the RLPA would not have a legal leg to stand on if the NRL did it as far as I am aware.