charge the researchers to have their submission reviewed and published,
As you point out, they are providing a valuable service. The research is peer-reviewed and published.
You can argue that they aren't needed in the modern peer-to-peer wikiresearch napster world, where anyone can just publish their research wherever they want.
If the researchers don't want their research behind the paywall, they are not forced into the transaction. They must be finding the value-add of the "peer-review and publish" significant enough to give up publishing rights elsewhere.