There are actual people working to curate the content, manage the community, etc. And as such, they charge an amount for the work they're doing. If you think the price does not meet the value, then you can... just not pay it.
Are people actually curating the content? The website sounds like the members are the ones suggesting books. It is unclear to me what managing needs to be done if the members are pairing up on their own.
Someone has to manage the membership fees, naturally.