I would not want that to be done either. I have experienced forums were a monthly number of posts had to be made for staying in. The policy has an extremely negative influence on the quality. People just posted to chalk up the required posts. Towards the end of the month that tended to render the forum unusable.
Anyway people who do not post, do not offend or bother anyone. I am also in a HAM forum where registering for mostly reading members is explicitly encouraged. It helps the owner to judge the reach of the forum.