Quoting is screwy again.
But in short, it would cause WBTS problems because it introduces accountability and criminal penalties into how some aspects of their child safeguarding policies are implemented. Not just at a local level, where local elders can be left to hang if it suits HQ, but also at circuit overseer and branch level too. Because the holy spirit sure as hell isn't going to be reading and writing DBS reports.
On cost, most volunteers get the check done for free. https://www.gov.uk/guidance/dbs-check-requests-guidance-for-employers#volunteer-applications