And besides, the information is either free or it isn't. Even if you don't register, you can still read everything that is being written. You just can't participate in the discussions. So, you aren't really paying for the information. You are paying for the right to post.
You're partially right. Because to get information better, members should have be able to discuss. Like Question - Answer discussion that is more effective when you can ask more if you need.
Better information could also be gotten without questioning and discussions. It all depends on individual differences. In my high school days, I had a genius friend who never questioned the teacher nor discuss with his mates but was very outstanding. Also, If you go to bookshop and buys a hardcopy book, I am sure you will have no one to question nor discuss with.
Some information are self explanatory, and even your intending questions might coincidentally be asked and answered by others. So, without posting one can actually gather enough information. Actually, the payers are paying for the right to post.